WorldWideScience
1

Ground and excited state electron transfer dynamics  

OpenAIRE

The focus of this work is the investigation of the factors controlling electron transfer in molecular electronic systems, in particular those affecting electron transfer to and from electronically excited states. To achieve this, a number of mono- and trimetallic osmium and ruthenium complexes were synthesised and characterised. Monolayers of an osmium polypyridyl complex bound to a platinum microelectrode via a ¿rara-l,2-bis-(4-pyridyl)ethylene bridge were formed to probe ground state elect...

Brennan, Jennifer L.

2002-01-01

2

Electronic excitation transfer in concentrated micelle solutions  

International Nuclear Information System (INIS)

Electronic excitation transport among interacting clusters of chromophores is investigated as a function of chromophores is investigated as a function of chromophore and cluster concentration. The technique of time-correlated single photon counting is employed to obtain time-resolved fluorescence depolarization data on aqueous octadecylrhodamine B/triton X-100 micelle solutions. The time-dependent fluorescence anisotropy, the energy transport observable, is directly compared to a theory developed to model this system. The theory is based on a first-order cumulant approximation to the solution of the transport master equation. The model depicts the micelles as monodisperse hard spheres with chromophores (octadecylrhodamine B) distributed about their surfaces. At low micelle concentration, the dynamics of excitation transfer depend only on internal micelle structure. At high micelle concentration excitation transfer occurs among chromophores on different micelles in addition to intramicelle transfer. The theoretical treatment provides nearly quantitative descriptions of the time and concentration dependence of the excitation transport. It correctly predicts the concentration at which intermicelle transfer becomes significant. In the low micelle concentration limit (energy transport confined to isolated micelles) the model having a Poisson distribution of chromophores works well for small ? ([chromophores]/[micelle]), but progressively worse as ? is increased. Followinsively worse as ? is increased. Following the literature, a chromophere interaction parameter (in the form of a two dimensional second virial coefficient) is used to skew the probe distribution. This enables the transport theory to reproduce the data for all the values of ? investigated and provides a determination of the second virial coefficient. 34 refs., 4 figs., 2 tabs

3

The role of excited Rydberg States in electron transfer dissociation.  

Science.gov (United States)

Ab initio electronic structure methods are used to estimate the cross sections for electron transfer from donor anions having electron binding energies ranging from 0.001 to 0.6 eV to each of three sites in a model disulfide-linked molecular cation. The three sites are (1) the S-S sigma(*) orbital to which electron attachment is rendered exothermic by Coulomb stabilization from the nearby positive site, (2) the ground Rydberg orbital of the -NH(3)(+) site, and (3) excited Rydberg orbitals of the same -NH(3)(+) site. It is found that attachment to the ground Rydberg orbital has a somewhat higher cross section than attachment to either the sigma orbital or the excited Rydberg orbital. However, it is through attachment either to the sigma(*) orbital or to certain excited Rydberg orbitals that cleavage of the S-S bond is most likely to occur. Attachment to the sigma(*) orbital causes prompt cleavage because the sigma energy surface is repulsive (except at very long range). Attachment to the ground or excited Rydberg state causes the S-S bond to rupture only once a through-bond electron transfer from the Rydberg orbital to the S-S sigma(*) orbital takes place. For the ground Rydberg state, this transfer requires surmounting an approximately 0.4 eV barrier that renders the S-S bond cleavage rate slow. However, for the excited Rydberg state, the intramolecular electron transfer has a much smaller barrier and is prompt. PMID:16599533

Sobczyk, Monika; Simons, Jack

2006-04-13

4

Electron transfer, ionization, and excitation in atomic collisions: Progress report  

International Nuclear Information System (INIS)

The fundamental processes of electron transfer, ionization, and excitation in ion-atom collisions are being studied at Penn State by Winter. (The related work of Alston, who recently came to Penn State, is not described here since he is not at present funded by DOE.) These collision processes are treated in the context of simple one- or two-electron systems in order to provide unambiguous results and reveal more clearly the collisional mechanisms. Three coupled-state calculations are being carried out over the present three-year period and are discussed here: a Sturmian-pseudostate study of electron transfer in collisions between protons and the hydrogenic ions He+, Li2+, Be3+, ...; a triple-center, atomic-state study of ionization in collisions between ? particles and H(ls) atoms and between protons and He+(ls) ions; and a coupled-state study of electron transfer and excitation in collisions between protons and neutral He atoms

5

Electron transfer, ionization, and excitation atomic collisions  

International Nuclear Information System (INIS)

Basic atomic-collision processes at intermediate and high energies are being studied theoretically at Penn State by Alston and Winter. In the high velocity regime, single-electron capture is treated using a high order multiple-scattering approach; extensive comparison with experiment and analysis of mechanisms have been made. Fitting the calculated amplitude with a simple analytic form, the asymptotic velocity dependence of the cross section is obtained. The effect on the capture amplitude of altering the inner part of the internuclear potential has also been explored. In the intermediate velocity regime, earlier work on collisions between protons and hydrogenic-ion targets using a coupled-state approach is being extended to the two-electron helium target. 29 refs

6

[Electron transfer, ionization, and excitation in atomic collisions  

International Nuclear Information System (INIS)

Fundamental processes of electron transfer, ionization, and excitation in ion-atom and ion-ion collisions are studied. Attention is focussed on one- and two-electron systems and, more recently, quasi-one-electron systems whose electron-target-ion core can be accurately modeled by one-electron potentials. The basic computational approaches can then be taken with few, if any, approximations, and the underlying collisional mechanisms can be more clearly revealed. At intermediate collision energies (e.g., proton energies for p-He+ collisions on the order of 100 kilo-electron volts), many electronic states are strongly coupled during the collision, a coupled-state approach, such as a coupled-Sturmian-pseudostate approach, is appropriate. At higher collision energies (million electron-volt energies) the coupling is weaker with, however, many more states being coupled together, so that high-order perturbation theory is essential

7

Electronic coulombic coupling of excitation-energy transfer in xanthorhodopsin.  

Science.gov (United States)

Electronic coupling of excitation-energy transfer (EET) in a retinal (RET) protein, xanthorhodopsin (xR), was studied theoretically. The protein, functioning as a light driven proton pump, contains a carotenoid antenna, salinixanthin (SXN), to collect light energy for an RET chromophore through EET. The pseudo-Coulombic interaction (PCI) between the donor SXN and the acceptor RET molecules was calculated by a transition density fragment interaction (TDFI) method, which overcomes difficulty arising in the evaluation of PCI in xR by a conventional dipole-dipole (dd) method, at the ab initio TDDFT/SAC-CI level of theory. The result nicely agrees with the experimentally observed PCI. To examine the correlation between the SXN-RET alignment and the EET efficiency, we computed PCIs for SXN conformations that are virtually generated around the protein. The calculation shows that the optimal SXN alignment for the maximally tuned efficiency of EET is attained in the native xR. PCI in another retinal protein, archaerhodopsin-2, which also binds a carotenoid but lacks EET activity, was also evaluated. The computed PCI is negligibly small, well explaining the lack of EET efficiency. PMID:19772318

Fujimoto, Kazuhiro J; Hayashi, Shigehiko

2009-10-14

8

Linear Energy Relationships in Ground State Proton Transfer and Excited State Proton-Coupled Electron Transfer.  

Science.gov (United States)

Proton-coupled electron transfer (PCET) processes are elementary chemical reaction involved in a broad range of radical and redox reactions. Elucidating fundamental PCET reaction mechanisms are thus of central importance for chemical and biochemical research. Here we use quantum chemical density functional theory (DFT), time-dependent density functional theory (TDDFT), and the algebraic diagrammatic-construction through second-order (ADC(2)) to study the mechanism, thermodynamic driving force effects and reaction barriers of both ground state proton transfer (pT) and photo-induced proton-coupled electron transfer (PCET) between nitrosylated phenyl-phenol compounds and hydrogen-bonded t-butylamine as an external base. We show that the obtained reaction barriers for the ground state pT reactions depend linearly on the thermodynamic driving force, with a Brønsted slope of 1 or 0. Photo-excitation leads to a PCET reaction, for which we find that the non-adiabatic excited state reaction barrier depend on the thermodynamic driving force with a Brønsted slope of ½. To support the mechanistic picture arising from the static potential energy surfaces, we perform additional molecular dynamics simulations on the excited state energy surface, in which we observe a spontaneous PCET between the donor and acceptor groups. Our findings suggest that a Brønsted analysis may distinguish the ground state pT and excited state PCET processes. PMID:25485993

Gámiz-Hernández, Ana-Patricia; Magomedov, Artiom; Hummer, Gerhard; Kaila, Ville R I

2014-12-01

9

Study of intermediates from transition metal excited-state electron-transfer reactions  

Energy Technology Data Exchange (ETDEWEB)

Conventional and fast-kinetics techniques of photochemistry, photophysics, radiation chemistry, and electrochemistry were used to study the intermediates involved in transition metal excited-state electron-transfer reactions. These intermediates were excited state of Ru(II) and Cr(III) photosensitizers, their reduced forms, and species formed in reactions of redox quenchers and electron-transfer agents. Of particular concern was the back electron-transfer reaction between the geminate pair formed in the redox quenching of the photosensitizers, and the dependence of its rate on solution medium and temperature in competition with transformation and cage escape processes. (DLC)

Hoffman, M.Z.

1992-07-31

10

Are electron-hole excitations responsible for the R^-4 dependence of excitation energy transfer (NSET) to a nanoparticle?  

CERN Document Server

We study the distance dependence of the rate of electronic excitation energy transfer from a dye molecule to a metal nanoparticle. Using the spherical jellium model, we evaluate the rates corresponding to the excitation of l = 1, 2, and 3 modes of the nanoparticle for the case of random but static averaging over all the orientations of the nanoparticle. Our calculation takes into account both the electron-hole pair and the collective modes of excitations of the nanoparticle. The rate follows the conventional R^-6 dependence at large distances while deviations from this behaviour are observed at shorter distances. The actual rates are found to be higher than the asymptotic R^-6 rates at shorter distances and have an R^-n dependence with n>6. Within the framework of the jellium model, it is not possible to attribute the experimentally observed R^-4 dependence of the rate to energy transfer to plasmons or e-h pair excitations of the nanoparticle.

Swathi, R S

2007-01-01

11

Excitation of the ligand-to-metal charge transfer band induces electron tunnelling in azurin  

Energy Technology Data Exchange (ETDEWEB)

Optical excitation of azurin blue copper protein immobilized on indium-tin oxide, in resonance with its ligand-to-metal charge transfer absorption band, resulted in a light-induced current tunnelling within the protein milieu. The related electron transport rate is estimated to be about 10{sup 5}?s{sup ?1}. A model based on resonant tunnelling through an azurin excited molecular state is proposed. The capability of controlling electron transfer processes through light pulses opens interesting perspectives for implementation of azurin in bio-nano-opto-electronic devices.

Baldacchini, Chiara [Biophysics and Nanoscience Centre, DEB-CNISM, Università della Tuscia, I-01100 Viterbo (Italy); Institute of Agro-environmental and Forest Biology, National Research Council, I-05010 Porano (Italy); Bizzarri, Anna Rita; Cannistraro, Salvatore, E-mail: cannistr@unitus.it [Biophysics and Nanoscience Centre, DEB-CNISM, Università della Tuscia, I-01100 Viterbo (Italy)

2014-03-03

12

Theoretical study on the electronic structure of triphenyl sulfonium salts: Electronic excitation and electron transfer processes  

Science.gov (United States)

Density functional theory (DFT) and Time Dependent DFT calculations on triphenyl sulfonium cation (TPS) and the salts of TPS with triflate, nonaflate, perfluoro-1-octanesulfonate and hexafluoro antimonate anions are presented. These systems are widely used as cationic photoinitiators and as electron ejection layer for polymer light-emitting diodes. While some differences exist in the electronic structure of the different salts, their lowest energy intense absorption maxima are calculated at nearly the same energy for all systems. The first excited state of TPS and of the TPS salts is dissociating. Electron addition to the TPS salts lowers their energy by 1.0-1.33 eV.

Petsalakis, Ioannis D.; Theodorakopoulos, Giannoula; Lathiotakis, Nektarios N.; Georgiadou, Dimitra G.; Vasilopoulou, Maria; Argitis, Panagiotis

2014-05-01

13

Excitational energy and photoinduced electron transfer reactions in Ge(IV) corrole–porphyrin hetero dimers  

International Nuclear Information System (INIS)

We have constructed hetero dimers by utilizing the axial bonding capabilities as well as known oxophilicity of Germanium(IV) ion of Germanium(IV) corroles as basal scaffolding unit and either free-base or ZnII porphyrin at axial position for the first time. Both the hetero dimers have been completely characterized by elemental analysis, UV–visible, proton nuclear magnetic resonance (1D and 1H–1H COSY) and fluorescence spectroscopies as well as electrochemical methods. The ground state properties indicate that there exists a minimum ?–? interactions between the macrocyclic units of these dyads. Excited state properties showed that there is an electronic energy transfer competing photoinduced electron transfer from singlet state of basal metalloid corrole to the axial porphyrin and a photoinduced electron transfer from excited state of axial porphyrin to the ground state of central metalloid corrole are possible. -- Highlights: ? • We have constructed hetero dimers based on corrole–porphyrin by utilizing axial position of Ge(IV) corrole. • There exists mimum ?–? interactions between the macrocyclic units. • The excited state properties indicate that both excitation energy transfer and photoinduced electron transfer reactions are possible in these hetero dimers

14

Excitational energy and photoinduced electron transfer reactions in Ge(IV) corrole–porphyrin hetero dimers  

Energy Technology Data Exchange (ETDEWEB)

We have constructed hetero dimers by utilizing the axial bonding capabilities as well as known oxophilicity of Germanium(IV) ion of Germanium(IV) corroles as basal scaffolding unit and either free-base or Zn{sup II} porphyrin at axial position for the first time. Both the hetero dimers have been completely characterized by elemental analysis, UV–visible, proton nuclear magnetic resonance (1D and {sup 1}H–{sup 1}H COSY) and fluorescence spectroscopies as well as electrochemical methods. The ground state properties indicate that there exists a minimum ?–? interactions between the macrocyclic units of these dyads. Excited state properties showed that there is an electronic energy transfer competing photoinduced electron transfer from singlet state of basal metalloid corrole to the axial porphyrin and a photoinduced electron transfer from excited state of axial porphyrin to the ground state of central metalloid corrole are possible. -- Highlights: ? • We have constructed hetero dimers based on corrole–porphyrin by utilizing axial position of Ge(IV) corrole. • There exists mimum ?–? interactions between the macrocyclic units. • The excited state properties indicate that both excitation energy transfer and photoinduced electron transfer reactions are possible in these hetero dimers.

Giribabu, Lingamallu, E-mail: giribabu@iict.res.in; Kandhadi, Jaipal; Kanaparthi, Ravi Kumar; Reeta, P. Silviya

2014-01-15

15

Toward control of electron transfer in donor-acceptor molecules by bond-specific infrared excitation.  

Science.gov (United States)

Electron transfer (ET) from donor to acceptor is often mediated by nuclear-electronic (vibronic) interactions in molecular bridges. Using an ultrafast electronic-vibrational-vibrational pulse-sequence, we demonstrate how the outcome of light-induced ET can be radically altered by mode-specific infrared (IR) excitation of vibrations that are coupled to the ET pathway. Picosecond narrow-band IR excitation of high-frequency bridge vibrations in an electronically excited covalent trans-acetylide platinum(II) donor-bridge-acceptor system in solution alters both the dynamics and the yields of competing ET pathways, completely switching a charge separation pathway off. These results offer a step toward quantum control of chemical reactivity by IR excitation. PMID:25525241

Delor, Milan; Scattergood, Paul A; Sazanovich, Igor V; Parker, Anthony W; Greetham, Gregory M; Meijer, Anthony J H M; Towrie, Michael; Weinstein, Julia A

2014-12-19

16

Study of intermediates from transition metal excited-state electron-transfer reactions  

Energy Technology Data Exchange (ETDEWEB)

During this period, conventional and fast-kinetics techniques of photochemistry, photophysics, radiation chemistry, and electrochemistry were used for the characterization of the intermediates that are involved in transition metal excited-state electron-transfer reactions. The intermediates of interest were the excited states of Ru(II) and Cr(III) photosensitizers, their reduced forms, and the species formed in the reactions of redox quenchers and electron-transfer agents. Of particular concern has been the back electron-transfer reaction between the geminate pair formed in the redox quenching of the photosensitizers, and the dependence of its rate on solution medium and temperature in competition with transformation and cage escape processes.

Hoffman, M.Z.

1991-12-31

17

Quenching of the excited state of hydrated Europium(III) ions by electron transfer  

Energy Technology Data Exchange (ETDEWEB)

This thesis explores the oxidation-reduction chemistry of the excited state of Eu(III) ions, *Eu{sub aq}{sup 3+}, in aqueous solutions. Evidence is presented for the quenching of *Eu{sup 3+} by reductive electron transfer. It is concluded that *Eu{sup 3+} is not a strong energy transfer reagent. The reactivity of *Eu{sub aq}{sup 3+} is compared with that of *UO{sub 2}{sup 2+}.

Christensen, K.

1993-08-01

18

Excited state electron transfer processes in solution and at solid surfaces  

Energy Technology Data Exchange (ETDEWEB)

The use of resonance Raman spectroscopy for the detection of the transient intermolecular electron transfer product generated in a reductive quenching of excited tris(2,2'-bipyridine)ruthenium (II) (Ru(bpy)[sub 3][sup 2+]) process is demonstrated. N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) is used as the electron donor. The leading edge of an 10 ns 532 nm pulse from a frequency-doubled Nd:YAG laser excitation source is used to generate Ru(bpy)[sub 3][sup 2+*], and the trailing edge of the pulse is used to detect the electron transfer product, TMPD[sup +]. Pulsed laser excitation of a covalently-linked chromophore-electron acceptor compound, pyridyltritolyporphyrin linked to dibenzylviologen, B[sub z2]V[sup 2+], (porphyrin-viologen, P-V[sup 2+]), in CH[sub 3]CN leads to intramolecular electron transfer quenching of the porphyrin singlet excited state within the laser pulsewidth. Quenching occurs by the reduction of the linked Bz[sub 2]V[sup 2+] to Bz[sub 2]V[sup 1+]. The direct detection of transient Bz[sub 2]V[sup 1+] was accomplished by resonance Raman spectroscopy. The same transient features were obtained from pulsed laser excitation of a mixture of the separate constituents of porphyrin (P) an dibenzylviologen in CH[sub 3]CN. Confirmation of the assignment of transient Bz[sub 2]V[sup 1+] comes from comparison of the spectra with the resonance Raman spectrum of an authentic sample of Bz[sub 2]V[sup 1+], and of electrochemically reduced P-V[sup 2+]. Fluorescence lifetime determinations for P-V[sup 2+] and P yield a rate constant for intramolecular electron transfer consistent with the ability to observe electron transfer within the laser pulsewidth. The results demonstrate that anchoring a molecular assembly on a SiO[sub 2] surface causes little perturbation of their photophysical properties and their excited state electron transfer process.

Yu, J.

1993-01-01

19

Fragment transition density method to calculate electronic coupling for excitation energy transfer  

Science.gov (United States)

A general approach, the Fragment Transition Density (FTD) scheme, is introduced to estimate electronic coupling for excitation energy transfer in a molecular system. Within this method, the excitation energies and transition densities of the system are used to derive the coupling matrix element. The scheme allows one to treat systems where exciton donor and acceptor are close together and their exchange interaction and orbital overlap are significant. The FTD method can be applied in combination with any quantum mechanical approach to treat excited states of general nature including single-, double-, and higher excitations. Using FTD approach, we derive excitonic couplings for several systems computed with the CIS, TD DFT and MS-CASPT2 methods. In particular, it is shown that the estimated coupling values in DNA ?-stacks are strongly affected by the short-range electronic interaction of adjacent nucleobases.

Voityuk, Alexander A.

2014-06-01

20

Fragment transition density method to calculate electronic coupling for excitation energy transfer  

International Nuclear Information System (INIS)

A general approach, the Fragment Transition Density (FTD) scheme, is introduced to estimate electronic coupling for excitation energy transfer in a molecular system. Within this method, the excitation energies and transition densities of the system are used to derive the coupling matrix element. The scheme allows one to treat systems where exciton donor and acceptor are close together and their exchange interaction and orbital overlap are significant. The FTD method can be applied in combination with any quantum mechanical approach to treat excited states of general nature including single-, double-, and higher excitations. Using FTD approach, we derive excitonic couplings for several systems computed with the CIS, TD DFT and MS-CASPT2 methods. In particular, it is shown that the estimated coupling values in DNA ?-stacks are strongly affected by the short-range electronic interaction of adjacent nucleobases

21

Electron transfer dynamics and excited state branching in a charge-transfer platinum(II) donor-bridge-acceptor assembly.  

Science.gov (United States)

A linear asymmetric Pt(ii) trans-acetylide donor-bridge-acceptor triad designed for efficient charge separation, NAP[triple bond, length as m-dash]Pt(PBu3)2[triple bond, length as m-dash]Ph-CH2-PTZ (), containing strong electron acceptor and donor groups, 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and phenothiazine (PTZ) respectively, has been synthesised and its photoinduced charge transfer processes characterised in detail. Excitation with 400 nm, ?50 fs laser pulse initially populates a charge transfer manifold stemming from electron transfer from the Pt-acetylide centre to the NAP acceptor and triggers a cascade of charge and energy transfer events. A combination of ultrafast time-resolved infrared (TRIR) and transient absorption (TA) spectroscopies, supported by UV-Vis/IR spectroelectrochemistry, emission spectroscopy and DFT calculations reveals a self-consistent photophysical picture of the excited state evolution from femto- to milliseconds. The characteristic features of the NAP-anion and PTZ-cation are clearly observed in both the TRIR and TA spectra, confirming the occurrence of electron transfer and allowing the rate constants of individual ET-steps to be obtained. Intriguingly, has three separate ultrafast electron transfer pathways from a non-thermalised charge transfer manifold directly observed by TRIR on timescales ranging from 0.2 to 14 ps: charge recombination to form either the intraligand triplet (3)NAP with 57% yield, or the ground state, and forward electron transfer to form the full charge-separated state (3)CSS ((3)[PTZ(+)-NAP(-)]) with 10% yield as determined by target analysis. The (3)CSS decays by charge-recombination to the ground state with ?1 ns lifetime. The lowest excited state is (3)NAP, which possesses a long lifetime of 190 ?s and efficiently sensitises singlet oxygen. Overall, molecular donor-bridge-acceptor triad demonstrates excited state branching over 3 different pathways, including formation of a long-distant (18 Å) full charge-separated excited state from a directly observed vibrationally hot precursor state. PMID:25361227

Scattergood, Paul A; Delor, Milan; Sazanovich, Igor V; Bouganov, Oleg V; Tikhomirov, Sergei A; Stasheuski, Alexander S; Parker, Anthony W; Greetham, Gregory M; Towrie, Michael; Davies, E Stephen; Meijer, Anthony J H M; Weinstein, Julia A

2014-12-21

22

Photo- and radiation chemical studies of intermediates involved in excited-state electron-transfer reactions  

International Nuclear Information System (INIS)

Excited-state inter- and intramolecular electron-transfer reactions lie at the heart of the most photochemical solar energy conversion schemes. The authors research, which has utilized the techniques of continuous and pulsed photolysis and radiolysis, has focused on three general aspects of these reactions involving transition metal coordination complexes and electron donor-acceptor complexes: i) the effect of solution medium on the properties and quenching of the excited states; ii) the control of the quantum yields of formation of redox products; iii) the mechanism by which reduced species interact with water to yield H2 homogeneously and heterogeneously. EDTA is among the most popular sacrificial electron donors used in model systems. Its role is to scavenge the oxidized form of the photosensitizer in order to prevent its rapid reaction with the reduced form of the electron relay species that results from the electron-transfer quenching of the excited photosensitizer. In systems involving MV2+, the radicals resulting from the oxidation of EDTA can eventually lead to the generation of a second equivalent of MV+; the reducing agent is believed to be a radical localized on the carbon atom alpha to the carboxylate group. The reaction of radiolytically-generated OH/H with EDTA produces this radical directly via H-abstraction or indirectly via deprotonation of the carbon atom adjacent to the nitrogen radical site in the oxidized amine moiety; it reduces MV2+ with rate constants of 2.8 x 109, 7.6 x 109, and 8.5 x 106M-1s-1 at pH 12.5, 8.3, and 4.7, respectively. Degradative decarboxylation of EDTA-radicals and their back electron-transfer reactions are enhanced in acidic solution causing the yield of MV+ to be severely diminished

23

Quenching of excited states of lanthanide complexes by electron and energy transfer processes  

International Nuclear Information System (INIS)

The interaction of the luminescent excited states of the Euaq3+ and Tbaq3+ ions, Eu and Tb complexes with M(CN)6z- complexes (M=Cr(III), FE(II), Co(III), Ru(II), or Os(II)) was studied in an aqueous solution by luminescence lifetime measurements. The lower than diffusion values obtained for the quenching constants of exergonic energy and electron transfer processes are attributed to poor electronic factors (nonadiabatic behavior). The results can be explained by a reasonable assumption that the precursor complex had a different structure depending on the lanthanide species involved. (author). 1 fig., 16 refs

24

Resonance energy transfer from a fluorescent dye molecule to plasmon and electron-hole excitations of a metal nanoparticle  

OpenAIRE

We study the distance dependence of the rate of electronic excitation energy transfer from a dye molecule to a metal nanoparticle. Using the spherical jellium model, we evaluate the rates corresponding to the excitation of l = 1, 2, and 3 modes of the nanoparticle. Our calculation takes into account both the electron-hole pair and the plasmon excitations of the nanoparticle. The rate follows conventional R^-6 dependence at large distances while small deviations from this beh...

Swathi, R. S.; Sebastian, K. L.

2007-01-01

25

Resonant Transfer Excitation Followed by Auger-Electron Emission for Ne-like Ions  

International Nuclear Information System (INIS)

In ion-atom (I/A) collision, resonant transfer excitation followed by Auger emission (RTEA) is a complementary process to resonant transfer excitation followed by X-rays (RTEX). In both processes positive ions are assumed to collide with molecular H2-target or atomic He-targets. RTEA and RTEX are related to the resonance excitation (RE) and dielectronic recombination (DR) in electron-ion collisions. The cross sections of RTEA and RE are related under the validity of the conditions of impulse approximation, in which the ionic projectiles must move with very high velocity. RTEA are calculated for Ne-like ions with L-shell excitation. The database generated for Auger and radiative decay rates in LS coupling scheme. RTEA processes proceed through formation of resonance R states. These R-states have a general form 2p5n1 l 1n2 l 2 ; n1 = 3, 4 and n2 ? 4. It is found that RTEA cross sections exhibit a one-peak behavior with a peak value 1.6 x 10-18 cm2 when it collides with H2 as a target. This peak value decreases to 0.488x10-18 cm2 in case of Se24+

26

Quenching of the excited state of hydrated europium(III) ions by electron transfer  

Science.gov (United States)

The quenching of *Eu 3+aq by several metal complexes and organic compounds takes place by electron transfer. The reactions are rather slow and only the most powerful reductants quench *Eu 3+ effectively. The most reactive quenchers were Ru(NH 3) 2+6, Fe(C 5Me 5) 2, and Fe(C 5H 5) 2. It was observed that Eu 2+ and Cr 2+ have similar reactivity with *Eu 3+. Quenching with N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) and N,N'-diphenyl-1,4-phenylenediamine (DPPD) yields the strongly absorbing radicals TMPD + rad and DPPD + rad , thus providing direct evidence for electron transfer. The slow rates of the *Eu 3+ reactions imply that the electron-exchange rate constant for *Eu 3+aq/Eu 2+aq is small, like that for the ground-state couple. The energy transfer quenching by metal ion complexes is also inefficient, as demonstrated by the lack of reactivity of non-reducing Mn 2+ and Co(NH 3) 3+6 complexes. Eu 3+ in the ground state reacts effectively with excited state biacetyl by energy transfer to form *Eu 3+.

Christensen, Kenneth M.; Bakac, Andreja; Espenson, James H.

1993-10-01

27

FAST TRACK COMMUNICATION: NO-? emissions from streamer discharges: direct electron impact excitation versus resonant energy transfer  

Science.gov (United States)

It has been established that production of NO-? emission in pulsed corona discharges is dominated by the energy transfer from N_2(A\\,^3\\! \\Sigma_u^+) to the NO ground state NO(X 2?r) while direct excitation by electron impact is negligible. However, recent studies suggest that the electron impact excitation plays a more important role. In this work, we report modelling results of NO-? emission associated with streamer discharges using two cross section data sets available in the literature. The first set was originally reported by Mojarrabi et al (1996 Phys. Rev. A 54 2977-82) and later updated by Brunger et al (2000 J. Phys. B: At. Mol. Opt. Phys. 33 80-19) the second set was published by Hayashi (1990 Nonequilibrium Processes in Partially Ionized Gases (NATO Advanced Science Institutes Series, Series B, Physics vol 220) ed M Capitelli and J N Bardsley (New York: Plenum) pp 333-40). According to the results, the role played by the electron impact excitation in the production of NO-? is drastically different when different cross sections are used. The results indicate that the first data set leads to better agreement with experimental measurements.

Liu, Ningyu; Pasko, Victor P.

2010-03-01

28

Intermolecular and Intramolecular Electron Transfer Processes from Excited Naphthalene Diimide Radical Anions.  

Science.gov (United States)

Excited radical ions are interesting reactive intermediates owing to powerful redox reactivities, which are applicable to various reactions. Although their reactivities have been examined for many years, their dynamics are not well-defined. In this study, we examined intermolecular and intramolecular electron transfer (ET) processes from excited radical anions of naphthalene-1,4,5,8-tetracarboxydiimide (NDI(•-)*). Intermolecular ET processes between NDI(•-)* and various electron acceptors were confirmed by transient absorption measurements during laser flash photolysis of NDI(•-) generated by pulse radiolysis. Although three different imide compounds were employed as acceptors for NDI(•-)*, the bimolecular ET rate constants were similar in each acceptor, indicating that ET is not the rate-determining step. Intramolecular ET processes were examined by applying femtosecond laser flash photolysis to two series of dyad compounds, where NDI was selectively reduced chemically. The distance dependence of the ET rate constants was described by a ? value of 0.3 Å(-1), which is similar or slightly smaller than the reported values for donor-acceptor dyads with phenylene spacers. Furthermore, by applying the Marcus theory to the driving force dependence of the ET rate constants, the electronic coupling for the present ET processes was determined. PMID:25397719

Fujitsuka, Mamoru; Kim, Sung Sik; Lu, Chao; Tojo, Sachiko; Majima, Tetsuro

2014-11-25

29

Competition and interplay of various intermolecular interactions in ultrafast excited-state proton and electron transfer reactions.  

Science.gov (United States)

The main features of the photoinduced kinetics of both ultrafast excited-state proton and electron transfer reactions that occur in the picosecond (ps) and femtosecond (fs) time domains are compared. Proton transfer (PT) reaction kinetics can be described in terms of several discrete values of rate coefficients in the form of polyexponential functions where each value of the rate coefficient can be attributed to a definite physical behavior of the reaction mechanism. In contrast, electron transfer (ET) reaction kinetics requires a consideration of a continuous distribution of rate coefficients. This difference can be related to structure of the ground-state reactant pairs for each reaction. Excited-state ET can occur at various configurations of reactant molecules and its rate reflects the fluctuations of the distances and orientations of these molecules. In contrast, excited-state PT requires preliminary formation of a ground-state H-bonded complex with definite structure where the reaction occurs after photoexcitation. PMID:25208189

Kuzmin, Michael G; Soboleva, Irina V; Ivanov, Vladimir L; Gould, Elizabeth-Ann; Huppert, Dan; Solntsev, Kyril M

2015-02-12

30

Ultrafast excited state dynamics of the perylene radical cation generated upon bimolecular photoinduced electron transfer reaction.  

Science.gov (United States)

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe(*+), generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump-pump-probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe(*+) was found to depend on the time delay between the first and second pump pulses, Deltat(12), i.e., on the "age" of the ion. At short Deltat(12), the GSR dynamics is biphasic, and at Deltat(12) larger than about 500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe(*+) remains biphasic independently of Deltat(12). The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis. PMID:16774195

Pagès, Stéphane; Lang, Bernhard; Vauthey, Eric

2006-06-22

31

Electron impact excitation of molecules  

International Nuclear Information System (INIS)

A brief overview on electron impact excitation of molecules is given with emphasis on elastic scattering, rotational, vibrational and electronic excitations. A few general remarks on dissociation and ionization are made. Recent developments in the electron-photon coincidence area are mentioned only for completeness. Practically no cross section data are available for inner-shell excitations and for electron collisons with excited molecules. Presently used experimental techniques and difficulties related to cross section measurements are briefly discussed, available differential, integral, momentum transfer and total electron scattering cross section data are surveyed, and experimental and theoretical results in the near threshold to few hundred eV impact energy range are compared. (Auth.)

32

Excited states, electron-transfer reactions, and intermediates in bacterial photosynthetic reaction centers  

Energy Technology Data Exchange (ETDEWEB)

The three-dimensional structure of a photosynthetic reaction center has recently been obtained at atomic resolution using x-ray crystallography by Deisenhofer, Epp, Miki, Huber, and Michel (J. Mol. Biol. 1984, 180, 385-398; Nature 1985, 318, 618-624). This breakthrough provides the fundamental structural information needed to understand the mechanisms of the initial energy- and electron-transfer steps in photosynthesis. The structure reveals the distances among the reactive bacteriochlorophylls and quinones as well as the location of all nearby solvent molecules, the amino acids of the reaction center protein. Thus, the reaction center provides a complex but well-defined solid-state reactive system for the study of fundamental physical and chemical processes with implications and applications well beyond this specific system. We review recent studies of the reaction intermediates and mechanism of electron transfer in which the energetics and reaction dynamics have been perturbed with external electric and magnetic fields. Electron-transfer mechanisms which have been proposed are reviewed critically in light of the available data, and electron transfer in the reaction center is compared with electron transfer in other biological and nonbiological systems.

Boxer, S.G.; Goldstein, R.A.; Lockhart, D.J.; Middendorf, T.R.; Takiff, L. (Stanford Univ., CA (USA))

1989-12-28

33

The form of electron-atom excitation amplitudes at high momentum transfers in the Faddeev-Watson approximation  

International Nuclear Information System (INIS)

A form of the off-shell Coulomb T matrix, which has a well defined on-shell limit, is used in the Faddeev-Watson multiple-scattering expansion for a direct three-body collision process. Using the excitation of atomic hydrogen by electron impact as an example, approximations to the second-order terms, which are valid for high momentum transfers of the incident electron, are derived. It is shown how the resulting asymptotic behaviour of the second-order Faddeev-Watson approximation is related to the high momentum transfer limit of the second Born approximation. The results are generalised to the excitation of more complex atoms. The asymptotic forms of the Faddeev-Watson and Born approximations are compared with other theories and with measurements of differential cross sections and angular correlation parameters for the excitation of H(2p) and He(21P). The results indicate that the Faddeev-Watson approximation converges more rapidly at high momentum transfers than does the Born approximation. (author)

34

Electron photodetachment from iodide in ionic liquids through charge-transfer-to-solvent band excitation.  

Science.gov (United States)

Solvation of iodide and electrons in an ionic liquid (N,N,N-trimethyl-n-propylammonium bis(trifluoromethanesulfonyl)imide; TMPA-TFSI) was studied through the absorption spectra of the charge-transfer-to-solvent (CTTS) state of iodide and of solvated electrons. The interaction between the TMPA cation and iodide was strong, whereas electrons were weakly solvated in TMPA-TFSI. We followed electron photodetachment from iodide to the ionic liquid and formation of the solvated electrons by observing absorption in the visible and near-infrared regions using a nanosecond laser flash photolysis method. The quantum yield of the photodetachment in TMPA-TFSI was estimated to be 0.34, which is much higher than that in a high-concentration aqueous salt solution previously reported. We also examined a reaction of the solvated electrons with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Bmim-TFSI) as a solute in TMPA-TFSI. The reaction rate was determined to be 5.3 x 10(8) M(-1) s(-1). The electrons before full solvation (dry electrons) reacted with Bmim cations efficiently. These observations suggest that the electrons in TMPA-TFSI can move easily before solvation. PMID:17474702

Katoh, Ryuzi; Yoshida, Yoichi; Katsumura, Yosuke; Takahashi, Kenji

2007-05-10

35

Exciton delocalization, charge transfer, and electronic coupling for singlet excitation energy transfer between stacked nucleobases in DNA: an MS-CASPT2 study.  

Science.gov (United States)

Exciton delocalization and singlet excitation energy transfer have been systematically studied for the complete set of 16 DNA nucleobase dimers in their ideal, single-strand stacked B-DNA conformation, at the MS-CASPT2 level of theory. The extent of exciton delocalization in the two lowest (?,?(*)) states of the dimers is determined using the symmetrized one-electron transition density matrices between the ground and excited states, and the electronic coupling is calculated using the delocalization measure and the energy splitting between the states [see F. Plasser, A. J. A. Aquino, W. L. Hase, and H. Lischka, J. Phys. Chem. A 116, 11151-11160 (2012)]. The calculated couplings lie between 0.05 eV and 0.14 eV. In the B-DNA conformation, where the interchromophoric distance is 3.38 Å, our couplings deviate significantly from those calculated with the transition charges, showing the importance of orbital overlap components for the couplings in this conformation. The calculation of the couplings is based on a two-state model for exciton delocalization. However, in three stacks with a purine in the 5(') position and a pyrimidine in the 3(') one (AT, GC, and GT), there is an energetically favored charge transfer state that mixes with the two lowest excited states. In these dimers we have applied a three-state model that considers the two locally excited diabatic states and the charge transfer state. Using the delocalization and charge transfer descriptors, we obtain all couplings between these three states. Our results are important in the context of DNA photophysics, since the calculated couplings can be used to parametrize effective Hamiltonians to model extended DNA stacks. Our calculations also suggest that the 5(')-purine-pyrimidine-3(') sequence favors the formation of charge transfer excited states. PMID:24606381

Blancafort, Lluís; Voityuk, Alexander A

2014-03-01

36

Exciton delocalization, charge transfer, and electronic coupling for singlet excitation energy transfer between stacked nucleobases in DNA: An MS-CASPT2 study  

Energy Technology Data Exchange (ETDEWEB)

Exciton delocalization and singlet excitation energy transfer have been systematically studied for the complete set of 16 DNA nucleobase dimers in their ideal, single-strand stacked B-DNA conformation, at the MS-CASPT2 level of theory. The extent of exciton delocalization in the two lowest (?,?{sup *}) states of the dimers is determined using the symmetrized one-electron transition density matrices between the ground and excited states, and the electronic coupling is calculated using the delocalization measure and the energy splitting between the states [see F. Plasser, A. J. A. Aquino, W. L. Hase, and H. Lischka, J. Phys. Chem. A 116, 11151–11160 (2012)]. The calculated couplings lie between 0.05 eV and 0.14 eV. In the B-DNA conformation, where the interchromophoric distance is 3.38 Å, our couplings deviate significantly from those calculated with the transition charges, showing the importance of orbital overlap components for the couplings in this conformation. The calculation of the couplings is based on a two-state model for exciton delocalization. However, in three stacks with a purine in the 5{sup ?} position and a pyrimidine in the 3{sup ?} one (AT, GC, and GT), there is an energetically favored charge transfer state that mixes with the two lowest excited states. In these dimers we have applied a three-state model that considers the two locally excited diabatic states and the charge transfer state. Using the delocalization and charge transfer descriptors, we obtain all couplings between these three states. Our results are important in the context of DNA photophysics, since the calculated couplings can be used to parametrize effective Hamiltonians to model extended DNA stacks. Our calculations also suggest that the 5{sup ?}-purine-pyrimidine-3{sup ?} sequence favors the formation of charge transfer excited states.

Blancafort, Lluís, E-mail: lluis.blancafort@udg.edu [Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus de Montilivi, 17071 Girona (Spain); Voityuk, Alexander A., E-mail: alexander.voityuk@icrea.cat [Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus de Montilivi, 17071 Girona (Spain); Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona 08010 (Spain)

2014-03-07

37

Exciton delocalization, charge transfer, and electronic coupling for singlet excitation energy transfer between stacked nucleobases in DNA: An MS-CASPT2 study  

International Nuclear Information System (INIS)

Exciton delocalization and singlet excitation energy transfer have been systematically studied for the complete set of 16 DNA nucleobase dimers in their ideal, single-strand stacked B-DNA conformation, at the MS-CASPT2 level of theory. The extent of exciton delocalization in the two lowest (?,?*) states of the dimers is determined using the symmetrized one-electron transition density matrices between the ground and excited states, and the electronic coupling is calculated using the delocalization measure and the energy splitting between the states [see F. Plasser, A. J. A. Aquino, W. L. Hase, and H. Lischka, J. Phys. Chem. A 116, 11151–11160 (2012)]. The calculated couplings lie between 0.05 eV and 0.14 eV. In the B-DNA conformation, where the interchromophoric distance is 3.38 Å, our couplings deviate significantly from those calculated with the transition charges, showing the importance of orbital overlap components for the couplings in this conformation. The calculation of the couplings is based on a two-state model for exciton delocalization. However, in three stacks with a purine in the 5? position and a pyrimidine in the 3? one (AT, GC, and GT), there is an energetically favored charge transfer state that mixes with the two lowest excited states. In these dimers we have applied a three-state model that considers the two locally excited diabatic states and the charge transfer state. Using the delocalization and charge transfer descriptors, we obtain all couplings between these three states. Our results are important in the context of DNA photophysics, since the calculated couplings can be used to parametrize effective Hamiltonians to model extended DNA stacks. Our calculations also suggest that the 5?-purine-pyrimidine-3? sequence favors the formation of charge transfer excited states

38

TDDFT calculations of electronic spectra of benzoxazoles undergoing excited state proton transfer.  

Science.gov (United States)

Energies and oscillator strengths of vertical transitions for various rotameric and tautomeric species of 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2,5-bis(2-benzoxazolyl)phenol (DBP) and 2,5-bis(2-benzoxazolyl)hydroquinone (BBHQ) have been calculated in the ground and first excited states with the use of TDDFT methods. The TDDFT results demonstrate good correspondence to the frequencies of absorption and fluorescence bands of the benzoxazoles reported for measurements in supersonic jets and solution, but fail to predict relative energies of the enol and keto tautomers of DBP and BBHQ in the excited state. Low intensity of the fluorescence bands attributed to the conformations of HBO and DBP that do not undergo excited state proton transfer is shown to be caused by low concentrations of the conformations in the ground state. For the three compounds large-amplitude twisting of the keto tautomer is found to be one of radiationless processes resulting in decrease of the fluorescence with a large Stokes shift. PMID:23475275

Syetov, Y

2013-07-01

39

Study of intermediates from transition metal excited-state electron-transfer reactions  

International Nuclear Information System (INIS)

This is the progress report for the period of January 1989 to March 1990 and contains brief summaries for the undertaken projects. The projects discussed are: cage escape yields in the quenching of Ru(II) photosensitizers, characterization of one-electron reduced Ru(II) complexes; application of Marcus theory to electron-transfer processes in Ru(II) complexes; synthesis and characterization of new Ru(II) photosensitizers; NMR study of ion-pairing in aqueous solutions containing Ru(bpy)32+; and reductive quenching of Ru(II) and Cr(III) complexes

40

Ultrafast Excited State Dynamics of the Perylene Radical Cation Generated upon Bimolecular Photoinduced Electron Transfer Reaction  

OpenAIRE

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe•+, generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump?pump?probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe•+ was found to depend on the time delay between the first and second pump pulses, ?t12, i.e., on the "age" of the ion. At short ?t12, the GSR dynamics is b...

Pages, Ste?phane; Lang, Bernhard Felix; Vauthey, Eric

2006-01-01

41

[Electron transfer, ionization, and excitation in atomic collisions]. Final technical report, June 15, 1986--June 14, 1998  

Energy Technology Data Exchange (ETDEWEB)

The research on theoretical atomic collisions that was funded at The Pennsylvania State University`s Wilkes-Barre Campus by DOE from 1986 to 1998 was carried out by Winger from 1986 to 1989 and by Winter and Alston from 1989 to 1998. The fundamental processes of electron transfer, ionization, and excitation in ion-ion, ion-atom, and, more recently, ion-molecule collisions were addressed. These collision processes were treated in the context of simple one-electron, quasi-one-electron, or two-electron systems in order to provide unambiguous results and reveal more clearly the collisional mechanisms. Winter`s work generally focused on the intermediate projectile-energy range corresponding to proton energies from about ten to a few hundred keV. In this velocity-matching energy range, the electron-transfer cross section reaches a peak, and many states, including electron-transfer and ionization states, contribute to the overall electron-cloud distribution and transition probabilities; a large number of states are coupled, and therefore perturbative approaches are generally inappropriate. These coupled-state calculations were sometimes also extended to higher energies to join with perturbative results. Alston concentrated on intermediate-energy asymmetric collision systems, for which coupling with the projectile is weaker, but many target states are included, and on high energies (MeV energies). Thus, while perturbation theory for electron transfer is valid, it is not adequate to first order. The studies by Winter and Alston described were often done in parallel. Alston also developed formal perturbative approaches not tied to any particular system. Materials studied included He{sup +}, Li{sup 2+}, Be{sup 3+}, B{sup 4+}, C{sup 5+}, and the H{sup +} + Na system.

NONE

1998-12-31

42

Ultrafast pump-probe spectroscopy of electronic excitation transfer in random molecular systems and in photosynthetic antennae  

Energy Technology Data Exchange (ETDEWEB)

Optical pump-probe (transient absorption) spectroscopy was used to study excitation energy transfer (EET) in random molecular systems and in antenna complexes of photosynthetic organisms. Energy transfer was first studied for Rhodamine 640 adsorbed on ZnO and fused silica surfaces at submonolayer coverage. Ground state recovery on fused silica was coverage-dependent due to excitation trapping by dye aggregates; on ZnO recovery was fast and independent of coverage. This can be explained by energy transfer into semiconductor surface states. Also, EET experiments were performed on solutions of Rhodamine 640 in glycerol using both absorption depolarization and the well-known technique of fluorescence depolarization using time-correlated single photon counting. Data from both techniques match within experimental error theoretical predictions, showing the validity of pump-probe techniques in studying EET. The absorption depolarization of the Q{sub x} electronic transition in BChl a-protein from P. aestuarii was monitored, and the anisotropy was found to decay to a constant, nonzero value due to nonrandom chromophore orientations. The timescale of depolarization was found to be consistent with transfer between delocalized exciton states. Results of experiments on the core antenna complex of photosystem I were remarkably similar, showing that local ordering exists in PS I. This also suggests that EET occurs between groups of chromophores in PS I similar to those in the BChl a-protein.

Causgrove, T.P.

1988-01-01

43

Study of intermediates from transition metal excited-state electron-transfer reactions. Final report, August 4, 1986--August 31, 1997  

Energy Technology Data Exchange (ETDEWEB)

The techniques of continuous photolysis and pulsed laser flash photolysis, continuous and pulse radiolysis, fast-scan cyclic voltammetry, and time-resolved fluorimetry have been used to examine intramolecular electron transfer within the solvent quenching cage, photodynamics of quenching of the excited states of transition-metal photosensitizers, the properties of excites states and one-electron reduced forms, ground- and excited-state interactions with solutes, and photoinduced oxidations of organic solutes in aqueous solution. The following specific areas were examined: (1) the parameters that govern the yields of redox products from excited-state electron-transfer quenching reactions; (2) the mediation of the properties of excited states and one-electron reduced forms by the ligands and the solution medium; (3) the effect of the interactions between the ground state of the complex and the solution components on the behavior of the excited state; (4) the yields of singlet oxygen from excited-state energy-transfer quenching by O{sub 2}; and (5) the oxidations of solutes by singlet oxygen, excited-state electron-transfer quenching, and free radicals. This report contains the abstracts of 50 publications describing the studies.

Hoffman, M.Z.

1997-12-31

44

Study of intermediates from transition metal excited-state electron-transfer reactions. [Annual] progress report, August 1, 1989--July 31, 1992  

Energy Technology Data Exchange (ETDEWEB)

Conventional and fast-kinetics techniques of photochemistry, photophysics, radiation chemistry, and electrochemistry were used to study the intermediates involved in transition metal excited-state electron-transfer reactions. These intermediates were excited state of Ru(II) and Cr(III) photosensitizers, their reduced forms, and species formed in reactions of redox quenchers and electron-transfer agents. Of particular concern was the back electron-transfer reaction between the geminate pair formed in the redox quenching of the photosensitizers, and the dependence of its rate on solution medium and temperature in competition with transformation and cage escape processes. (DLC)

Hoffman, M.Z.

1992-07-31

45

Correlations of excited electrons  

International Nuclear Information System (INIS)

Inelastic collisions of an electron with an atom or molecule at energies within several eV of an excitation or ionization threshold, lead to the temporary formation of a complex whose available energy is shared in comparable amounts between the incident electron and the one being excited. Analogous complexes are formed when two electrons are excited simultaneously by incident light or by an analogous external agent. The properties of such complexes, including the rate and modality of their prompt or delayed decomposition, depend on the correlation of the two excited electrons. Their study is reviewed with emphasis on achieving a graphical description of the correlations

46

Chemical Physics Electrons and Excitations  

CERN Document Server

A full understanding of modern chemistry is impossible without quantum theory. Since the advent of quantum mechanics in 1925, a number of chemical phenomena have been explained, such as electron transfer, excitation energy transfer, and other phenomena in photochemistry and photo-physics. Chemical bonds can now be accurately calculated with the help of a personal computer. Addressing students of theoretical and quantum chemistry and their counterparts in physics, Chemical Physics: Electrons and Excitations introduces chemical physics as a gateway to fields such as photo physics, solid-state ph

Larsson, Sven

2012-01-01

47

Excited singlet (S1) state interactions of calixarenes with chloroalkanes: A combination of concerted and stepwise dissociative electron transfer mechanism  

Science.gov (United States)

Both steady-state and time-resolved studies in acetonitrile (ACN) solutions show that the excited singlet (S1) states of calixarenes (CX) undergo quenching by chloroalkanes (CA). It has been revealed by characterizing the Cl ions in the photolyzed CX-CA systems in ACN solutions that the quenching occurs due to dissociative electron transfer (DET) mechanism, whereby a C-Cl bond of the CAs undergoes dissociation on acceptance of an electron from excited CX. The bimolecular quenching constants (kq) in the present systems were correlated with the free energy changes for the concerted DET reactions based on a suitable DET theory. Such a correlation results in the recovery of an intramolecular reorganization energy, which is substantially lower to account for the C-Cl bond dissociation energy of the CAs. Comparing present results with those of an another donor-acceptor system (e.g., biphenyldiol-CA systems) where a concerted DET mechanism is applicable, it is inferred that in CX-CA systems both concerted and stepwise DET mechanisms operate simultaneously. It is proposed that the interaction of excited CXs with encaged CAs follows the stepwise mechanism whereas that with the out of cage CAs follows the concerted mechanism.

Mohanty, J.; Pal, H.; Nayak, S. K.; Chattopadhyay, S.; Sapre, A. V.

2002-12-01

48

Dipole-Dipole Electron Excitation Energy Transfer in the System CdSe/ZnS Quantum Dot - Eosin in Butyral Resin Matrix  

Science.gov (United States)

The electron excitation energy transfer from CdSe/ZnS quantum dots to eosin molecules in the polymer matrix of butyral resin is investigated. The main characteristics of energy transfer are determined. By means of luminescence microscopy and correlation spectroscopy methods we found that quantum dots in the polymer are in an aggregate state.

Myslitskaya, N. A.; Samusev, I. G.; Bryukhanov, V. V.

2014-11-01

49

Charge Transfer Mechanism and Spatial Density Correlation of Electronic States of Excited Zinc (3d9 Films  

Directory of Open Access Journals (Sweden)

Full Text Available In material science, half filled 3d orbital of transition metals is essentially an important factor controlling characteristics of alloys and compounds. This paper presents a result of the challenge of excitation of inner-core electron system with long lifetime of zinc films. The advanced zinc films with excited inner-core electron, 3dn (n = 9, 8. We report experimental results of XPS measurements of 9 points in the sample along vertical direction, respectively. The most pronounced futures are existence of satellites, which are about 4 eV higher than the main lines. According to the charge transfer mechanism proposed by A. Kotani and K. Okada, it was clarified that the origins of these peaks are c3d9L for the main peak and c3d9 for the satellite, respectively. From the energy difference, ?EB, and peak intensity ratio, I+/I-, between c3d9 and c3d10L, the energy for charge transfer, ?, and mixing energy, T, were estimated. In the region where the intensity of c3d10L becomes large, ? becomes small, 1.2 ?dc = 5.5 eV and Udd = 5.5 eV. In the analysis along vertical direction, intensity profile of Zn3d9 showed odd functional symmetry and that of Zn3d10L showed even functional symmetry. Only the intensity profile of C1s (288 eV showed the same spatial correlation with Zn3d9. In our experiment, the sample also showed high mobility of the constituting elements. These suggest that charge conservation in excited zinc atom suggests combination between Zn3d9 and C2-.

Li Chen

2014-04-01

50

Quenching reactions of electronically excited atoms  

International Nuclear Information System (INIS)

The two-body, thermal quenching reactions of electronically excited atoms are reviewed using excited states of Ar, Kr, and Xe atoms as examples. State-specific interstate relaxation and excitation-transfer reactions with atomic colliders are discussed first. These results then are used to discuss quenching reactions of excited-state atoms with diatomic and polyatomic molecules, the latter have large cross sections, and the reactions can proceed by excitation transfer and by reactive quenching. Excited states of molecules are not considered; however, a table of quenching rate constants is given for six excited-state molecules in an appendix

51

Description of electron transfer in the ground and excited states of organic donor–acceptor systems by single-reference and multi-reference density functional methods  

International Nuclear Information System (INIS)

Electron transfer in the ground and excited states of a model donor–acceptor (D–A) system is investigated using the single-reference and multi-reference density functional theory (DFT) methods. To analyze the results of the calculations, a simple two-site multi-reference model was derived that predicts a stepwise electron transfer in the S0 state and a wave-like dependence of the S1 electron transfer on the external stimulus. The standard single-reference Kohn-Sham (KS) DFT approach and the time-dependent DFT (TDDFT) method failed to describe the correct dependence of the S0 and S1 electron transfer on the external electric field applied along the donor–acceptor system. The multi-reference DFT approach, the spin-restricted ensemble-referenced KS (REKS) method, was able to successfully reproduce the correct behavior of the S0 and S1 electron transfer on the applied field. The REKS method was benchmarked against experimentally measured gas phase charge transfer excitations in a series of organic donor–acceptor complexes and displayed its ability to describe this type of electronic transitions with a very high accuracy, mean absolute error of 0.05 eV with the use of the standard range separated density functionals. On the basis of the calculations undertaken in this work, it is suggested that the non-adiabatic coupling between the S0 and S1 states may interfere with the electron transfer in a weakly coupled donor–acceptor system. It is also suggested that the electronic excitation of a D+–A? system may play a dual role by assisting the further electron transfer at certain magnitudes of the applied electric field and causing the backward transfer at lower electric field strengths

52

Ultrafast pump-probe spectroscopy of electronic excitation transfer in random molecular systems and in photosynthetic antennae  

Energy Technology Data Exchange (ETDEWEB)

Optical pump-probe (transient absorption) spectroscopy was used to study excitation energy transfer (EET) in random molecular systems and in antenna complexes of photosynthetic organisms. Energy transfer was first studied for Rhodamine 640 adsorbed on ZnO and fused silica surfaces at submonolayer coverage. Also, EET experiments were performed on solutions of Rhodamine 640 in glycerol using both absorption depolarization and the well-known technique of fluorescence depolarization using time-correlated single photon counting. Data from both techniques match within experimental error theoretical predictions, showing the validity of pump-probe techniques in studying EET. The absorption depolarization of the Q/sub x/ electronic transitions in BChl a-protein from P. aestuarii was monitored, and the anisotropy was found to decay to a constant, nonzero value due to nonrandom chromophore orientations. The timescale of depolarization was found to be consistent with transfer between delocalized exciton states. Results of experiments on the core antenna complex of photosystem I were remarkably similar. 279 refs., 52 figs., 9 tabs.

Causgrove, T.P.

1989-02-01

53

Excitation energy transfer in the photosystem I  

Energy Technology Data Exchange (ETDEWEB)

Photosystem I is a multimeric pigment protein complex in plants, green alage and cyanobacteria that functions in series with Photosystem II to use light energy to oxidize water and reduce carbon dioxide. The Photosystem I core complex contains 96 chlorophyll a molecules and 22 carotenoids that are involved in light harvesting and electron transfer. In eucaryotes, PSI also has a peripheral light harvesting complex I (LHCI). The role of specific chlorophylls in excitation and electron transfer are still unresolved. In particular, the role of so-called bridging chlorophylls, located between the bulk antenna and the core electron transfer chain, in the transfer of excitation energy to the reaction center are unknown. During the past funding period, site directed mutagenesis has been used to create mutants that effect the physical properties of these key chlorophylls, and to explore how this alters the function of the photosystem. Studying these mutants using ultrafast absorption spectroscopy has led to a better understanding of the process by which excitation energy is transferred from the antenna chlorophylls to the electron transfer chain chlorophylls, and what the role of connecting chlorophylls and A_0 chlorophylls is in this process. We have also used these mutants to investigate whch of the central group of six chlorophylls are involved in the primary steps of charge separation and electron transfer.

Webber, Andrew N

2012-09-25

54

Electronic excited state redox properties for BODIPY dyes predicted from Hammett constants: estimating the driving force of photoinduced electron transfer.  

Science.gov (United States)

Here we formulate equations based solely on empirical Hammett substituent constants to predict the redox potentials for the electronic excited state of boron-dipyrromethene (BODIPY) dyes. We utilized computational, spectroscopic, and electrochemical techniques toward characterizing the effect of substitution at the positions C2, C6, and C8 of the 1,3,5,7-tetramethyl BODIPY core. Working with a library of 100 BODIPY dyes, we found that highest occupied molecular orbital (HOMO) energies calculated at the B3LYP 6-31g(d) level correlated linearly with the Hammett ?m value for substituents at position C8 and with Hammett ?p values for substituents at positions C2 and C6. In turn, we observed that LUMO energies correlated linearly with Hammett ?p at position C8 and with Hammett ?m at positions C2 and C6. Focusing on a subset of 26 dyes for which reduction potentials were either previously available or measured herein and ranged from -1.84 to -0.52 V (a full 1.3 V), we found a linear relationship between redox potentials in acetonitrile and HOMO and lowest unoccupied molecule orbital (LUMO) energies determined via density functional theory (DFT). A linear correlation was thus ultimately established between redox potentials in acetonitrile and Hammett substituent constants. Combining this with equations derived for the linear relationship existing between the zero vibrational energy of the excited BODIPY and Hammett substituent constants enabled us to provide the parameters toward predicting the oxidizing/reducing power of photoexcited 1,3,5,7,-tetramethyl BODIPY dyes in their singlet excited state. PMID:25066755

Lincoln, Richard; Greene, Lana E; Krumova, Katerina; Ding, Zhutian; Cosa, Gonzalo

2014-11-13

55

Monte Carlo simulations of excitation and electron transfer in grana membranes.  

Science.gov (United States)

Time-resolved fluorescence measurements on grana membranes with instrumental response function of 3ps reveal faster excitation dynamics (120ps) than those reported previously. A possible reason for the faster decay may be a relatively low amount of "extra" LHCII trimers per reaction center of Photosystem II. Monte Carlo modeling of excitation dynamics in C2S2M2 form of PSII-LHCII supercomplexes has been performed using a coarse grained model of this complex, constituting a large majority of proteins in grana membranes. The main factor responsible for the fast fluorescence decay reported in this work was the deep trap constituted by the primary charge separated state in the reaction center (950-1090cm(-1)). This value is critical for a good fit, whereas typical hopping times between antenna polypeptides (from ~4.5 to ~10.5ps) and reversible primary charge separation times (from ~4 to ~1.5ps, respectively) are less critical. Consequently, respective mean migration times of excitation from anywhere in the PSII-LHCII supercomplexes to reaction center range from ~30 to ~80ps. Thus 1/4-2/3 of the ~120-ps average excitation lifetime is necessary for the diffusion of excitation to reaction center, whereas the remaining time is due to the bottle-neck effect of the trap. Removal of 27% of the Lhcb6 apoprotein pool by mutagenesis of DEG5 gene caused the acceleration of the excitation decay from ~120 to ~100ps. This effect may be due to the detachment of LHCII-M trimers from PSII-LHCII supercomplexes, accompanied by deepening of the reaction center trap. PMID:25524819

Gibasiewicz, Krzysztof; Adamiec, Ma?gorzata; Luci?ski, Robert; Giera, Wojciech; Che?miniak, Przemys?aw; Szewczyk, Sebastian; Sipi?ska, Weronika; G?ów, Edyta; Karolczak, Jerzy; van Grondelle, Rienk; Jackowski, Grzegorz

2015-03-01

56

Influence of macromolecule main chain structure on electron excitation energy transfer in carbazole-containing polymers  

International Nuclear Information System (INIS)

Efficiencies of energy transfer from singlet excitons to rubrene in poly-N-epoxypropylcarbazole (PEPC) and from triplet excitons to pyrene in poly-N-vinylcarbazole (PVC) were found to increase on heating from 5 to 295 K. At the same time, the transfer efficiency in PVC at rubrene concentration 2*10-3 M decreased in the range T = 100-295 K, which correlated with a rise in the intensity of the fluorescence band of the matrix sandwich-like excimers.However, the transfer efficiency increased upon increasing the concentration, which caused the maximum donor-acceptor distance to approach the critical radius of the Forster model. The observed effects were explained by the fact that the free-path lengths of singlet excitons in PEPC and triplet excitons in PVC increased with increasing temperature and, correspondingly, the probability of them approaching the acceptor within distances where energy transfer was possible increased.Singlet-exciton self-trapping at excimer-forming sites and formation of sandwich-like excimers competed in PVC with energy transfer to rubrene. (author)

57

Intermediate vibronic coupling in charge transfer states: comprehensive calculation of electronic excitations in sexithiophene crystal.  

Science.gov (United States)

A comprehensive theory of linear vibronic coupling in a coupled manifold of Frenkel and charge-transfer states in an infinite molecular crystal is presented and applied for sexithiophene. The approach, valid in the intermediate-coupling regime, includes up to three-particle terms of the Philpott expansion, with the vibronic wavefunctions represented in the Lang-Firsov basis. As a stringent test, the scheme is used to reproduce the complete set of available sexithiophene absorption and electroabsorption spectra within a unified theoretical framework. The input is based primarily on independent calculations and to some extent on independent experiments, with explicit fitting contained within the limits set by the estimated inherent errors of a priori parameter estimates. Reasonably good quantitative agreement with experimental spectra is achieved. The results resolve some existing interpretational ambiguities and expose some peculiarities of electric field effect on vibronic eigenstates of Frenkel parentage, highlighting the role of charge-transfer interactions. PMID:21682523

Stradomska, Anna; Kulig, Waldemar; Slawik, Micha?; Petelenz, Piotr

2011-06-14

58

Study of intermediates from transition metal excited-state electron-transfer reactions. Progress report, August 1, 1989--December 31, 1991  

Energy Technology Data Exchange (ETDEWEB)

During this period, conventional and fast-kinetics techniques of photochemistry, photophysics, radiation chemistry, and electrochemistry were used for the characterization of the intermediates that are involved in transition metal excited-state electron-transfer reactions. The intermediates of interest were the excited states of Ru(II) and Cr(III) photosensitizers, their reduced forms, and the species formed in the reactions of redox quenchers and electron-transfer agents. Of particular concern has been the back electron-transfer reaction between the geminate pair formed in the redox quenching of the photosensitizers, and the dependence of its rate on solution medium and temperature in competition with transformation and cage escape processes.

Hoffman, M.Z.

1991-12-31

59

Nonradiative triplet-singlet transfer of electronic excitation energy between dye molecules in the vicinity of the silver-film surface  

Science.gov (United States)

Nonradiative triplet-singlet transfer of electronic-excitation energy between molecules of organic dyes (erythrosine and methylene blue) in a polymer film that is deposited on the surface of a silver layer is experimentally studied. It is demonstrated that the energy-transfer efficiency in such a system is lower than the efficiency in the samples without metal layers. The results of the proposed mathematical model are in qualitative agreement with the experimental data.

Kislov, D. A.; Kucherenko, M. G.

2014-11-01

60

Laser induced picosecond electron transfer with participation of excited states for porphyrins covalently linked with electron acceptor  

Science.gov (United States)

The dynamics and pathways of relaxation processes in meso-ortho-nitrophenyl substituted octaethylporphyrins OEP-OEPPh(o-NO II) and PdOEP-Ph(o-NO II) occurring with the participation of the S I and T I states was studied in polar Ph((dimethylformamide) and nonpolar (toluene) solvents at 295 and 77 K using pico- and nanosecond laser kinetic spectroscopy. At 295 K, steric interactions between bulky ?-alkyl substituents and ortho-nitro groups of meso-phenyl in these compounds create optimal conditions for overlapping of molecular orbitals of the porphyrin macrocycle (donor) and NO II-group (acceptor), thus leading to an efficient photoinduced electron transfer (PET). For free-base OEP-Ph(o-NO II), PET occurs only via the porphyrin S I state within 40 ps (dimethylformamide) and 125 ps (toluene), whereas the competing intersystem crossing S I ~~>T I is low probable. For metallocomplex PdOEP-Ph(o-NO II), PET involves both S I and T I states. In the last case, the direct PET from the T I state to CT state also occurs within picosecond range (20 and 46 ps for dimethylformamide and toluene, respectively, at 295 K). Rate constants for PET with participation of T1 states are by 3-5 times smaller with respect to those found for PET occurring via the S I state. For both compounds, the observed long-lived component (250-700 ns) in decays of the transient T I-T n absorption is due to the recombination processes of radical-ion pairs whose lifetime decreases with an increase of the surrounding polarity and is hardly dependent on the presence of molecular oxygen in the solution. For both compounds, PET is completely absent in rigid solutions at 77 K.

Sagun, Evgenii I.; Zenkevich, Eduard I.; Knyukshto, Valentin N.; Shulga, Alexander M.; Tikhomirov, Sergei A.

2007-06-01

61

Optimal initiation of electronic excited state mediated intramolecular H-transfer in malonaldehyde by UV-laser pulses  

Science.gov (United States)

Optimally controlled initiation of intramolecular H-transfer in malonaldehyde is accomplished by designing a sequence of ultrashort (~80 fs) down-chirped pump-dump ultra violet (UV)-laser pulses through an optically bright electronic excited [ S 2 ( ? ? ?)] state as a mediator. The sequence of such laser pulses is theoretically synthesized within the framework of optimal control theory (OCT) and employing the well-known pump-dump scheme of Tannor and Rice [D.J. Tannor, S.A. Rice, J. Chem. Phys. 83, 5013 (1985)]. In the OCT, the control task is framed as the maximization of cost functional defined in terms of an objective function along with the constraints on the field intensity and system dynamics. The latter is monitored by solving the time-dependent Schrödinger equation. The initial guess, laser driven dynamics and the optimized pulse structure (i.e., the spectral content and temporal profile) followed by associated mechanism involved in fulfilling the control task are examined in detail and discussed. A comparative account of the dynamical outcomes within the Condon approximation for the transition dipole moment versus its more realistic value calculated ab initio is also presented.

Nandipati, K. R.; Singh, H.; Nagaprasad Reddy, S.; Kumar, K. A.; Mahapatra, S.

2014-12-01

62

Enhanced electronic excitation energy transfer between dye molecules incorporated in nano-scale media with apparent fractal dimensionality  

Science.gov (United States)

In the present study, we analyze the efficiency of Electronic Excitation Energy Transfer (EEET) between two dyes, an energy donor (D) and acceptor (A), concentrated in structurally heterogeneous media (surfactant micelles, liposomes, and porous SiO2 matrices). In all three cases, highly effective EEET in pairs of dyes has been found and cannot be explained by Standard Förster-type theory for homogeneous solutions. Two independent approaches based on the analysis of either the D relative quantum yield () or the D fluorescence decay have been used to study the deviation of experimental results from the theoretical description of EEET process. The observed deviation is quantified by the apparent fractal distribution of molecules parameter . We conclude that the highly effective EEET observed in the nano-scale media under study can be explained by both forced concentration of the hydrophobic dyes within nano-volumes and non-uniform cluster-like character of the distribution of D and A dye molecules within nano-volumes.

Yefimova, Svetlana L.; Rekalo, Andrey M.; Gnap, Bogdan A.; Viagin, Oleg G.; Sorokin, Alexander V.; Malyukin, Yuri V.

2014-09-01

63

Electronic excitations in Guanine quadruplexes.  

Science.gov (United States)

Guanine rich DNA strands, such as those encountered at the extremities of human chromosomes, have the ability to form four-stranded structures (G-quadruplexes) whose building blocks are guanine tetrads. G-quadruplex structures are intensively studied in respect of their biological role, as targets for anticancer therapy and, more recently, of their potential applications in the field of molecular electronics. Here we focus on their electronic excited states which are compared to those of non-interacting mono-nucleotides and those of single and double stranded structures. Particular emphasis is given to excited state relaxation processes studied by time-resolved fluorescence spectroscopy from femtosecond to nanosecond time scales. They include ultrafast energy transfer and trapping of ??* excitations by charge transfer states. The effect of various structural parameters, such as the nature of the metal cations located in the central cavity of G-quadruplexes, the number of tetrads or the conformation of the constitutive single strands, are examined. PMID:24563011

Changenet-Barret, Pascale; Hua, Ying; Markovitsi, Dimitra

2015-01-01

64

The effect of thermal motion on the electron localization in metal-to-ligand charge transfer excitations in [Fe(bpy)3](2+).  

Science.gov (United States)

Accurate electronic structure calculations of the lowest excited states have been performed on twenty snapshots of a molecular dynamics simulation of [Fe(bpy)3](2+) dissolved in water. The thermal motion distorts the structure of the complex from its average D3 symmetry, causing the localization on one bipyridine ligand of the excited electron in the metal-to-ligand charge transfer (MLCT) state. The excitation energy is about 0.25 eV lower than that for the delocalized description of the MLCT state and is in good agreement with experiments. The composition of the MLCT band is carefully analyzed and the effect of thermal motion on the mechanism of light-induced spin crossover is discussed. PMID:25360630

Domingo, Alex; Sousa, Carmen; de Graaf, Coen

2014-12-21

65

Study of intermediates from transition metal excited-state electron-transfer reactions: Progress report, August 1, 1986--December 31, 1988  

International Nuclear Information System (INIS)

During the period covered by this progress report, the examination of intermediates from transition metal excited-state electron-transfer reactions followed certain well-defined themes; the techniques of conventional and time-resolved spectrofluorimetry, continuous and pulsed laser flash photolysis, and continuous and pulse radiolysis were employed. Research involves Ru(II) complexes and Cr(III) complexes. 23 refs., 2 tabs

66

Zinc porphyrin-re(i) bipyridyl-fullerene triad: synthesis, characterization, and kinetics of the stepwise electron-transfer processes initiated by visible excitation.  

Science.gov (United States)

A new triad system featuring one zinc porphyrin and one fullerene moieties attached to a central redox-active Re(I) connector was obtained in remarkable yield by cleverly exploiting a facile two-step synthesis. Detailed description and discussion on the characterization of this multicomponent system and of its parent free-base analogue are presented, along with a kinetic study of the stepwise electron-transfer processes occurring upon visible excitation. PMID:25523017

Cavigli, Paolo; Da Ros, Tatiana; Kahnt, Axel; Gamberoni, Marta; Indelli, Maria Teresa; Iengo, Elisabetta

2015-01-01

67

Resonant transfer and excitation by crystal channeling  

International Nuclear Information System (INIS)

A transition amplitude for resonant transfer and excitation (RTE) in a crystal channel is derived and compared with observations of Belkacem et al. [Phys. Rev. Lett. 64, 380 (1990)] of titanium ions channeled in a thin gold crystal. Using a one-dimensional tight-binding model of the gold lattice, the RTE cross section is shown to be proportional to the Compton profile of atomic gold multiplied by a crystal-structure factor, analogous to multislit interference of light with diffraction. It is suggested that Belkacem et al. observed a single (principal) maximum with a very narrow width determined by the crystal's Fermi energy. It is found that the observed shift in the RTE peak, relative to that expected for a quasifree crystal electron, can be accounted for by a change in the Coulomb energy of the transferred electron caused by the highly charged ''impurity'' ion prior to transfer

68

Resonant transfer and excitation by crystal channeling  

Energy Technology Data Exchange (ETDEWEB)

A transition amplitude for resonant transfer and excitation (RTE) in a crystal channel is derived and compared with observations of Belkacem {ital et} {ital al}. (Phys. Rev. Lett. 64, 380 (1990)) of titanium ions channeled in a thin gold crystal. Using a one-dimensional tight-binding model of the gold lattice, the RTE cross section is shown to be proportional to the Compton profile of {ital atomic} gold multiplied by a crystal-structure factor, analogous to multislit interference of light with diffraction. It is suggested that Belkacem {ital et} {ital al}. observed a single (principal) maximum with a very narrow width determined by the crystal's Fermi energy. It is found that the observed shift in the RTE peak, relative to that expected for a quasifree crystal electron, can be accounted for by a change in the Coulomb energy of the transferred electron caused by the highly charged impurity'' ion prior to transfer.

Feagin, J.M. (Fakultaet fuer Physik, Universitaet Freiburg, 7800 Freiburg (Gemany)); Wanser, K.H. (Department of Physics, California State University, Fullerton, Fullerton, California (USA))

1991-10-01

69

The electronic structure and charge transfer excited states of the endohedral trimetallic nitride C80 (Ih) fullerenes-Zn-tetraphenyl porphyrin dyads.  

Science.gov (United States)

Endohedral fullerenes offer the possibility of tuning their properties through a choice of the endohedral unit. The Sc3N@C80 fullerene is the most abundant fullerene after C60 and C70. Recently, Sc3N@C80 has been tested for light harvesting properties with encouraging results. In this work, we study the electronic structure of three endohedral fullerene-Zn tetraphenyl porphyrin complexes using density functional theory. The binding between the components in these complexes arises due to van der Waals interaction. A fragment orbital analysis is carried out to examine the interaction between the two components which shows that a small charge transfer occurs in the ground state from the ZnTPP to the fullerenes and that the orientation of the Sc3N plane affects the ground state charge transfer. The charge transfer excited state energies are calculated using our perturbative delta-SCF method. A comparison with earlier calculations shows that the charge transfer excitation energy increases as C60-ZnTPP < C70-ZnTPP < Sc3N@C80-ZnTPP < Y3N@C80-ZnTPP. The orientation of the endohedral unit does not influence the excitation energy in the donor-acceptor complexes. PMID:25631359

Basurto, Luis; Amerikheirabadi, Fatemeh; Zope, Rajendra; Baruah, Tunna

2015-02-10

70

Excited-state proton coupled electron transfer between photolyase and the damaged DNA through water wire: a photo-repair mechanism.  

Science.gov (United States)

The photolyase enzyme absorbs blue light to repair damaged DNA through a cyclic electron transfer reaction. A description of the underlying mechanism has proven to be a challenging issue for both experimental and theoretical studies. In the present work, combined CASPT2//CASSCF/AMBER (QM/MM) calculations have been performed for damaged DNA in photolyase. A proton-coupled electron transfer (PCET) mechanism has been determined for restoring cyclobutane pyrimidine dimer (CPD) to two normal thymine bases by irradiation of photolyase. A well-defined water wire between FADH(-) and CPD was determined as a bridge to assist the PCET process within FADH(-) and thereby trigger the forward electron transfer to CPD. The subsequent CPD splitting and the alternation of the H-bond pattern proceed in a concerted way, which makes the productive backward electron transfer occur on an ultrafast timescale. A local minimum of SCT((1)??*)-LMin was identified on the pathway of the futile backward electron transfer (BET), which is stabilized by the strong H-bond interaction between the water wire and CPD. As a result, the futile BET process is endothermic by ?18.0 kcal mol(-1), which is responsible for a 2.4 ns timescale inferred experimentally for the futile BET process. Besides the unbiased interpretation for the majority of the experimental findings, the present study provides a new excited-state PCET mechanism, which leads to a significant step toward a deeper understanding of the photo-repair process of damaged-DNA by the photolyase enzyme. PMID:25341360

Wang, Hongjuan; Chen, Xuebo; Fang, Weihai

2014-12-14

71

Theoretical study of intermolecular energy transfer involving electronically excited molecules: He(1S) + H2(B 1?/sub u/+)  

International Nuclear Information System (INIS)

To further understanding of gas phase collision dynamics involving electronically-excited molecules, a fully quantum mechanical study of He + H2(B 1?/sub u/+) was undertaken. Iterative natural orbital configuration interaction (CI) calculations were performed to obtain the interaction potential between He and H2(B 1?/sub u/+). The potential energy surface (PES) is highly anisotropic and has a van der Waals well of about 0.03 eV for C/sub 2v/ approach. Avoided PES crossings occur with He + H2(E,F 1?/sub g/+) and with He + H2(X 1?/sub g/+) and cause a local maximum and a deep minimum in the He + H2(B 1?/sub u/+) PES, respectively. The crossing with He + H2(X 1?/sub g/+) provides a mechanism for fluorescence quenching. The computed CI energies were combined with previous multi-reference double excitation CI calculations and fit with analytic functions for convenience in scattering calculations. Accurate dipole polarizabilities and quadrupole moment of H2(B 1?/sub u/+) were computed for use in the multipole expansion, which is the analytic form of the long-range PES. 129 refs., 28 figs., 35 tabs

72

Electron attachment to excited molecules  

International Nuclear Information System (INIS)

Studies on electron attachment to molecules rotationally/vibrationally excited thermally or via infrared-laser excitation showed that the effect of internal energy of a molecule on its electron attachment properties depends on the mode--dissociative or nondissociative--of electron attachment. They quantified the effect of the internal energy of the molecule on the rate of destruction (by autodissociation or by autodetachment) of its parent transient anion. Generally, increases in ro-vibrational molecular energy increase the cross section for dissociative electron attachment and decrease the effective cross section for parent anion formation due mainly to increased autodetachment. These findings and their understanding are discussed. A discussion is given, also, of recent investigations of electron attachment to electronically excited molecules, especially photoenhanced dissociative electron attachment to long- and short-lived excited electronic states of molecules produced directly or indirectly by laser irradiation. These studies showed that the cross sections for dissociative electron attachment to electronically excited molecules usually are many orders of magnitude larger than those for the ground-state molecules. The new techniques that have been developed for such studies are briefly described also

73

Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis  

Science.gov (United States)

The scheme of energy levels previously proposed to describe dual excitation and emission associated to excited state intramolecular proton transfer (ESIPT) of some hydroxyanthraquinones (HAQ's) has been made more quantitative in the present paper. The zero-point energy and the frequency of the ?OH mode for the HAQ's have been calculated on the basis of the Lippincott-Schroeder double-minimum potential for the O-H⋯O hydrogen bond. The second derivative absorption (D2) spectra show that the vibrational structures of the electronic excited state of HAQ's giving rise to ESIPT are characterized by the progression of the ?OH stretching mode. The ?OH mode in the ground state is observed as a very strong band in the vibrational structure of the short wavelength emission for HAQ's showing ESIPT. The combined resonance Raman band assignment of four hydroxyanthraquinones and transform analysis show that the visible transition involves the hydrogen bonded cycle and induces proton transfer in the excited state in most cases. On the basis of the isotopic effects, some vibrations of the hydrogen bonded cycle, namely the ?C=O, ?C=O, ?COH, and ?OH modes, have been identified. The transform method, including the combined analysis of the absorption and D2 spectra in terms of sum-over-states, was checked by directly deriving the displacement parameters (Franck-Condon factors) of 1,4-DHAQ from the high resolution free-jet spectrum. The values of the displacement parameters of the ?OH mode are quite large for the HAQ's showing ESIPT, while are negligible for 1,4-DHAQ. High values of the displacement parameters for the other vibrations of the hydrogen bonded cycle were found for all HAQ's.

Marzocchi, Mario P.; Mantini, Anna R.; Casu, Maurizio; Smulevich, Giulietta

1998-01-01

74

From free radicals to electronically excited species.  

Science.gov (United States)

Biologically/medically important compounds, when metabolized, can generate free radicals from which electrically excited products--often in the triplet state--are generated. Peroxidases are particularly apt to catalyze such processes, which usually entail oxidations by electron transfer. In the latter case, the chemiluminescence may derive from peroxyl and alkoxyl radicals or excited states derived from dioxetanes. Besides peroxidases, prostaglandin-H synthase and lipoxygenase may catalyze the formation of excited carbonyls. The pronounced similarity in the chemical behavior and reactivity of radicals and excited species derives in part from the biradical nature of the latter. Usually in analyzing the biological effects of xenobiotics, only radicals and/or reactive ground state products have been considered. However, in such processes the generation of excited species is possible, which should be tested for by direct and/or sensitized emission or by photochemical transformation. PMID:7635351

Cilento, G; Adam, W

1995-07-01

75

Faster photoinduced electron transfer in a diluted mixture than in a neat donor solvent: effect of excited-state H-bonding.  

Science.gov (United States)

In a neat electron-donating solvent (in this case aniline), photoinduced electron transfer (PET) from the solvent to an excited acceptor (e.g. a coumarin fluorophore) may be anticipated to be the most efficient because of the close contact of the acceptor with many donors. Addition of an inert component would most likely retard the PET process by replacing some donors from the neighbourhood of the acceptors. Surprisingly, we found dramatic acceleration of PET (6-10 fold enhancement compared to neat aniline), for coumarin 102 (C102) dissolved in a binary mixture of aniline and an inert solvent (cyclohexane or toluene). The PET induced fluorescence follows an anomalous trend against the mole fraction of aniline (XAN); first quenches up to certain XAN (0.075 for cyclohexane; 0.13 for toluene), thereafter, enhances with increase in XAN. Although the non-interacting component cannot directly participate in the PET process, it may modulate C102-aniline H-bonding association by changing the polarity of the medium or by disrupting the aniline-aniline H-bond. The study clearly illustrates the dominant role of hydrogen bonding in activating the electron transfer rate where standard thermodynamics predicts very weak donor-acceptor interaction. PMID:24562294

Barman, Nabajeet; Singha, Debabrata; Sahu, Kalyanasis

2014-04-01

76

The electron-electron interaction studied in strong central fields by resonant transfer and excitation with H-like U ions. - Angular distribution of hypersatellite and satellite radiation emitted after resonant and excitation into U91+ ions  

International Nuclear Information System (INIS)

Electron-electron interaction is studied in the strongest possible atomic fields (Z.? => 1) in the presence of only two electrons. A quasi-free electron from a hydrogen gas target is resonantly captured into a Lj subshell of a fast H-like U91+ ion by simultaneous excitation of the strongly bound K electron also into a Lj' subshell of the projectile, with j and j' the total angular momenta of 1/2 or 3/2 for the electron of concern. This resonant capture and excitation process, KLjLj'-RTE, is mediated by electron-electron interaction. It is equivalent to dielectronic recombination (DR) in ion-electron collisions and leads to a doubly excited He-like U90+** ion, which stabilizes - almost exclusively - via the emission of two successive K X-rays, first a K hypersatellite (K?i-H) and then a K satellite (K?i'-S) transition. The K X-ray emission characteristics associated with one-electron capture in collisions of U91+ ions with a hydrogen target is studied for the three resonance groups of the KLjLj'-RTE and one off-resonance energy, i.e. in the energy range between 100 and 135 MeV/u. The total cross section for the first resonance group KL1/2L1/2 confirms the importance of the Breit contribution to the interaction. The angular distribution for the K?2-H transition (j=1/2) is isotropic in the projectile system whereas the K?1-H transition (j=3/2) indicates a strong alignment for the 3/2 electrons in the doubly excited states for the second resonance group KL1/2L3/2. The experimental results are in agreement with fully relativistic calculations including the generalized Breit interaction. (orig.)

77

Excitation energy transfer and electron-vibrational coupling in phycobiliproteins of the cyanobacterium Acaryochloris marina investigated by site-selective spectroscopy.  

Science.gov (United States)

In adaption to its specific environmental conditions, the cyanobacterium Acaryochloris marina developed two different types of light-harvesting complexes: chlorophyll-d-containing membrane-intrinsic complexes and phycocyanobilin (PCB) - containing phycobiliprotein (PBP) complexes. The latter complexes are believed to form a rod-shaped structure comprising three homo-hexamers of phycocyanin (PC), one hetero-hexamer of phycocyanin and allophycocyanin (APC) and probably a linker protein connecting the PBPs to the reaction centre. Excitation energy transfer and electron-vibrational coupling in PBPs have been investigated by selectively excited fluorescence spectra. The data reveal a rich spectral substructure with a total of five low-energy electronic states with fluorescence bands at 635nm, 645nm, 654nm, 659nm and a terminal emitter at about 673 nm. The electronic states at ~635 and 645 nm are tentatively attributed to PC and APC, respectively, while an apparent heterogeneity among PC subunits may also play a role. The other fluorescence bands may be associated with three different isoforms of the linker protein. Furthermore, a large number of vibrational features can be identified for each electronic state with intense phonon sidebands peaking at about 31 to 37cm?¹, which are among the highest phonon frequencies observed for photosynthetic antenna complexes. The corresponding Huang-Rhys factors S fall in the range between 0.98 (terminal emitter), 1.15 (APC), and 1.42 (PC). Two characteristic vibronic lines at about 1580 and 1634cm?¹ appear to reflect CNH? and CC stretching modes of the PCB chromophore, respectively. The exact phonon and vibrational frequencies vary with electronic state implying that the respective PCB chromophores are bound to different protein environments. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy. PMID:24560813

Gryliuk, G; Rätsep, M; Hildebrandt, S; Irrgang, K-D; Eckert, H-J; Pieper, J

2014-09-01

78

Electron transfer in peptides.  

Science.gov (United States)

In this review, we discuss the factors that influence electron transfer in peptides. We summarize experimental results from solution and surface studies and highlight the ongoing debate on the mechanistic aspects of this fundamental reaction. Here, we provide a balanced approach that remains unbiased and does not favor one mechanistic view over another. Support for a putative hopping mechanism in which an electron transfers in a stepwise manner is contrasted with experimental results that support electron tunneling or even some form of ballistic transfer or a pathway transfer for an electron between donor and acceptor sites. In some cases, experimental evidence suggests that a change in the electron transfer mechanism occurs as a result of donor-acceptor separation. However, this common understanding of the switch between tunneling and hopping as a function of chain length is not sufficient for explaining electron transfer in peptides. Apart from chain length, several other factors such as the extent of the secondary structure, backbone conformation, dipole orientation, the presence of special amino acids, hydrogen bonding, and the dynamic properties of a peptide also influence the rate and mode of electron transfer in peptides. Electron transfer plays a key role in physical, chemical and biological systems, so its control is a fundamental task in bioelectrochemical systems, the design of peptide based sensors and molecular junctions. Therefore, this topic is at the heart of a number of biological and technological processes and thus remains of vital interest. PMID:25619931

Shah, Afzal; Adhikari, Bimalendu; Martic, Sanela; Munir, Azeema; Shahzad, Suniya; Ahmad, Khurshid; Kraatz, Heinz-Bernhard

2015-02-10

79

Synergy Effects of Electric and Magnetic Fields on Locally Excited-State Fluorescence of Photoinduced Electron Transfer Systems in a Polymer Film  

Science.gov (United States)

Photoluminescence of electron donor-acceptor pairs that show photoinduced electron transfer (PIET) has been measured in a polymer film under simultaneous application of electric field and magnetic field. Fluorescence emitted from the locally excited state (LE fluorescence) of 9-methylanthracene (MAnt) and pyrene (Py) is quenched by an electric field in a mixture of 1,3-dicyanobenzene (DCB) with MAnt or Py, indicating that PIET from the excited state of MAnt or Py to DCB is enhanced by an electric field. Simultaneous application of electric and magnetic fields enhances the reverse process from the radical-ion pair produced by PIET to the LE fluorescent state of MAnt or Py. As a result, the electric-field-induced quenching of the LE fluorescence is reduced by application of the magnetic fields. Thus, the synergy effect of electric and magnetic fields is observed on the LE fluorescence of MAnt or Py. Exciplex fluorescence spectra resulting from PIET can be obtained by analyzing the field effects on photoluminescence spectra, even when the exciplex fluorescence is too weak to be determined from the steady-state or time-resolved photoluminescence spectra at zero field.

Awasthi, Kamlesh; Iimori, Toshifumi; Ohta, Nobuhiro

2009-09-01

80

Charge transfer and excitation in high-energy ion-atom collisions  

International Nuclear Information System (INIS)

Coincidence measurements of charge transfer and simultaneous projectile electron excitation provide insight into correlated two-electron processes in energetic ion-atom collisions. Projectile excitation and electron capture can occur simultaneously in a collision of a highly charged ion with a target atom; this process is called resonant transfer and excitation (RTE). The intermediate excited state which is thus formed can subsequently decay by photon emission or by Auger-electron emission. Results are shown for RTE in both the K shell of Ca ions and the L shell of Nb ions, for simultaneous projectile electron loss and excitation, and for the effect of RTE on electron capture

81

'Horizontal' thermal equilibrium due to excitation transfer between excited states of neutral He in transient plasma  

International Nuclear Information System (INIS)

The populations of the levels and the intensities of the spectral lines emitted by neutral helium in a plasma is influenced by excitation transfer between excited levels through collisions with electrons even at very low electron densities. These processes are studied theoretically and experimentally with the following conclusions: (a) Excitation transfer is important for intensities of spectral lines emitted from plasma where the electron density exceeds a few times 1016 m-3. (b) With increasing electron density, the relative populations of levels with the same main and spin quantum number approach that determined by their statistical weight. This could be regarded as a ''horizontal'' thermal equilibrium, developed when the helium levels are very far from thermal equilibrium in other respects

82

Phonons and charge-transfer excitations in HTS superconductors  

International Nuclear Information System (INIS)

Some of the experimental and theoretical evidence implicating phonons and charge-transfer excitations in HTS superconductors is reviewed. It is suggested that superconductivity may be driven by a synergistic interplay of (anharmonic) phonons and electronic degrees of freedom (e.g., charge fluctuations, excitons). 47 refs., 5 figs

83

Charge Transfer Excitations in Insulating Copper Oxides  

CERN Document Server

A semi-quantitative cluster approach is developed to describe the charge transfer (CT) electron-hole excitations in insulating cuprates in a rather wide energy range up to 10- 15 eV. It generalizes the Zhang-Ng (ZN) model of CT excitons by considering the complete set of Cu3d and O2p orbitals within the CuO4 embedded molecular cluster method and by introducing one-center (intra-center) Frenkel-like and two-center (inter-center) excitons. Special attention is paid to the transition matrix element effects both in optical and electron energy loss spectra (EELS). In the latter case we obtain the momentum dependence of matrix elements both for intra-center and inter-center transitions. We are able to give a semi-quantitative description of the optical and EELS spectra for a large number of 0D (like CuB2O4), 1D (Sr2CuO3) and 2D (like Sr2CuO2Cl2) insulating cuprates in a unifying manner. By comparing our analysis with the experimental data we find that the CT gap in insulating cuprates is determined by nearly degene...

Moskvin, A S; Hayn, R; Malek, J

2005-01-01

84

Nonadiabatic anharmonic electron transfer  

Energy Technology Data Exchange (ETDEWEB)

The effect of an inner sphere, local mode vibration on an electron transfer is modeled using the nonadiabatic transition probability (rate) expression together with both the anharmonic Morse and the harmonic oscillator potential. For an anharmonic inner sphere mode, a variational analysis uses harmonic oscillator basis functions to overcome the difficulties evaluating Morse-model Franck-Condon overlap factors. Individual matrix elements are computed with the use of new, fast, robust, and flexible recurrence relations. The analysis therefore readily addresses changes in frequency and/or displacement of oscillator minimums in the different electron transfer states. Direct summation of the individual Boltzmann weighted Franck-Condon contributions avoids the limitations inherent in the use of the familiar high-temperature, Gaussian form of the rate constant. The effect of harmonic versus anharmonic inner sphere modes on the electron transfer is readily seen, especially in the exoergic, inverted region. The behavior of the transition probability can also be displayed as a surface for all temperatures and values of the driving force/exoergicity {Delta}=-{Delta}G. The temperature insensitivity of the transfer rate is clearly seen when the exoergicity equals the collective reorganization energy ({Delta}={Lambda}{sub s}) along a maximum ln (w) vs. {Delta} ridge of the surface. The surface also reveals additional regions for {Delta} where ln (w) appears to be insensitive to temperature, or effectively activationless, for some kinds of inner sphere contributions.

Schmidt, P. P. [Molecular Physics Research, 6547 Kristina Ursula Court, Falls Church, Virginia 22044 (United States)

2013-03-28

85

Electronic excitation in bulk and nanocrystalline alkali halides  

Science.gov (United States)

The lowest energy excitations in bulk alkali halides are investigated by considering five different excited state descriptions. It is concluded that excitation transfers one outermost halide electron in the fully ionic ground state to the lowest energy vacant s orbital of one closest cation neighbour to produce the excited state termed dipolar. The excitation energies of seven salts were computed using shell model description of the lattice polarization produced by the effective dipole moment of the excited state neutral halogen-neutral metal pair. Ab initio uncorrelated short-range inter-ionic interactions computed from anion wavefunctions adapted to the in-crystal environment were augmented by short-range electron correlation contributions derived from uniform electron-gas density functional theory. Dispersive attractions including wavefunction overlap damping were introduced using reliable semi-empirical dispersion coefficients. The good agreement between the predicted excitation energies and experiment provides strong evidence that the excited state is dipolar. In alkali halide nanocrystals in which each ionic plane contains only four ions, the Madelung energies are significantly reduced compared with the bulk. This predicts that the corresponding intra-crystal excitation energies in the nanocrystals, where there are two excited states depending on whether the halide electron is transferred to a cation in the same or in the neighbouring plane, will be reduced by almost 2 eV. For such an encapsulated KI crystal, it has been shown that the greater polarization in the excited state of the bulk crystal causes these reductions to be lowered to a 1.1 eV-1.5 eV range for the case of charge transfer to a neighbouring plane. For intra-plane charge transfer the magnitude of the polarization energy is further reduced thus causing the excitation in these encapsulated materials to be only 0.2 eV less than in the bulk crystal.

Bichoutskaia, Elena; Pyper, Nicholas C.

2012-11-01

86

Charge transfer excitations from excited state Hartree-Fock subsequent minimization scheme  

Energy Technology Data Exchange (ETDEWEB)

Photoinduced charge-transfer processes play a key role for novel photovoltaic phenomena and devices. Thus, the development of ab initio methods that allow for an accurate and computationally inexpensive treatment of charge-transfer excitations is a topic that nowadays attracts a lot of scientific attention. In this paper we extend an approach recently introduced for the description of single and double excitations [M. Tassi, I. Theophilou, and S. Thanos, Int. J. Quantum Chem. 113, 690 (2013); M. Tassi, I. Theophilou, and S. Thanos, J. Chem. Phys. 138, 124107 (2013)] to allow for the description of intermolecular charge-transfer excitations. We describe an excitation where an electron is transferred from a donor system to an acceptor one, keeping the excited state orthogonal to the ground state and avoiding variational collapse. These conditions are achieved by decomposing the space spanned by the Hartree-Fock (HF) ground state orbitals into four subspaces: The subspace spanned by the occupied orbitals that are localized in the region of the donor molecule, the corresponding for the acceptor ones and two more subspaces containing the virtual orbitals that are localized in the neighborhood of the donor and the acceptor, respectively. Next, we create a Slater determinant with a hole in the subspace of occupied orbitals of the donor and a particle in the virtual subspace of the acceptor. Subsequently we optimize both the hole and the particle by minimizing the HF energy functional in the corresponding subspaces. Finally, we test our approach by calculating the lowest charge-transfer excitation energies for a set of tetracyanoethylene-hydrocarbon complexes that have been used earlier as a test set for such kind of excitations.

Theophilou, Iris, E-mail: i.theophilou@fz-juelich.de [Peter Grunberg Institut (PGI) Forschungszentrum Jülich, D-52425 Jülich (Germany); Tassi, M.; Thanos, S. [Institute for Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, ‘Demokritos’ National Center for Scientific Research, 15310 Athens (Greece)

2014-04-28

87

Electron paramagnetic resonance of photocatalytic reaction which involve electron transfer  

Energy Technology Data Exchange (ETDEWEB)

Transient radicals generated under photocatalytic reactions in a polar solvent and their electronically spinned polarization were discussed under UV irradiation by using EPR and N2 gas pulse laser time-divided EPR. The reaction is a reaction of electron transfer from such amines as DABCO or electron donor molecules of SO3{sup -} to such electron accepting compounds as 1,4-benzoquinone and maleic anhydride under the presence of photocatalysts (triple photosensitizers) such as benzophenone and xanthone (XT). Spin polarized cation radicals of DABCO and radical anions of XT were detected in association with one electron transfer. A triple mechanism lies in the spinned polarization of both radicals, and transient XT in the triple state begin the electron transfer reaction. Photo-excited XT acts as a photocatalyst in one electron transfer reaction, the triple XT turns into an electron accepting body, and the transient anion radicals of XT become the electron donor. The XT(S) acts as a photocatalyst in the inter-molecular electron transfer from amine (D) to quinone (A). Its reaction is expressed by the following formula: D + S{sup *} + A{yields}D{sup dot +}+S+A{sup dot -} (where S{sup *} denotes a photo-excited state). 53 refs., 21 figs., 3 tabs.

Kaise, M. [National Institute of Materials and Chemical Research, Tsukuba (Japan)

1995-10-13

88

Cold transfer between deformed, Coulomb excited nuclei  

International Nuclear Information System (INIS)

The scattering system 162Dy ? 116Sn has been examined at energies in the vicinity of the Coulomb barrier using the Heidelberg-Darmstadt Crystal Ball spectrometer combined with 5 Germanium-CLUSTER detectors. In order to study pairing correlations as a function of angular momentum cold events were selected in the 2n stripping channel by identifying and suppressing the dominant hot part of the transfer with the Crystal Ball. The CLUSTER detectors with their high ?-efficiency were used to identify the transfer channel and to resolve individual final states. Cross sections for the population of individual yrast states in a cold transfer reaction have been measured for the first time indicating the strong influence of higher transfer multipolarities. At small surface distances Coulomb-nuclear interferences were found to be responsible for the stronger decline of the population of higher yrast states in the transfer channel as compared to the Coulex channel. As a preparatory study for 2n transfer measurements between high spin yrast states in the backbending region of deformed nuclei the Coulomb excitation process in the crossing region of two bands in 162Dy has been analyzed. The gross properties of the measured population probabilities could be interpreted in a simple band mixing model. (orig.)

89

Quantum Plasmonics: Electron transfer processes  

Science.gov (United States)

Plasmon energies can be tuned across the spectrum by simply changing the geometrical shape of a nanostructure. Plasmons can efficiently capture incident light and focus it to nanometer sized hotspots which can enhance electronic and vibrational excitations in nearby structures.[1] Another important but still relatively unexplored property of plasmons, is that they can be efficient sources of hot energetic electrons which can transfer into nearby structures and induce a variety of processes. This process is a quantum mechanical effect: the decay of plasmon quanta into electron-hole pairs. I will discuss how plasmon induced hot electrons can be used in various applications: such as to induce chemical reactions in molecules physisorbed on a nanoparticle surface;[2] to inject electrons directly into the conduction band of a nearby substrate;[3] and to induce local doping of a nearby graphene sheet.[4] References [1] N.J. Halas et al., Adv. Mat. 24(2012)4842 [2] R. Huschka et al., JACS 133(2011)12247; S. Mukherjee et al. TBP 2012 [3] M. W. Knight et al., Science 332(2011)702, Z.Y. Fang et al., NL 12(2012)3808 [4] Z.Y. Fang et al., ACS Nano 6(2012)10.1021/nn304028b

Nordlander, Peter

2013-03-01

90

Photoinduced electron transfer in a tris(2,2'-bipyridine)-C60-ruthenium(II) dyad: evidence of charge recombination to a fullerene excited state  

International Nuclear Information System (INIS)

A novel fulleropyrrolidine covalently linked to a tris(2,2'-bipyridine)-ruthenium(II) complex is presented. While electrochemical investigations suggest the absence of any ground-state interaction between ruthenium and fullerence chromophores, photoinduced optical absorption studies clearly show that electron transfer occurs to afford the Ru3+-C60.- pair with characteristic fullerene radical anion band at ?max=1040 nm. (orig.)

91

Nuclear reorganization barriers to electron transfer  

International Nuclear Information System (INIS)

The nuclear barrier to electron transfer arises from the need for reorganization of intramolecular and solvent internuclear distances prior to electron transfer. For reactions with relatively small driving force (''normal'' free-energy region) the nuclear factors and rates increase as intrinsic inner-shell and outer-shell barriers decrease; this is illustrated by data for transition metal complexes in their ground electronic states. By contrast, in the inverted free-energy region, rates and nuclear factors decrease with decreasing ''intrinsic'' barriers; this is illustrated by data for the decay of charge-transfer excited states. Several approaches to the evaluation of the outer-shell barrier are explored in an investigation of the distance dependence of the nuclear factor in intramolecular electron-transfer processes. 39 refs., 14 figs., 3 tabs

92

Nuclear reorganization barriers to electron transfer  

Energy Technology Data Exchange (ETDEWEB)

The nuclear barrier to electron transfer arises from the need for reorganization of intramolecular and solvent internuclear distances prior to electron transfer. For reactions with relatively small driving force (''normal'' free-energy region) the nuclear factors and rates increase as intrinsic inner-shell and outer-shell barriers decrease; this is illustrated by data for transition metal complexes in their ground electronic states. By contrast, in the inverted free-energy region, rates and nuclear factors decrease with decreasing ''intrinsic'' barriers; this is illustrated by data for the decay of charge-transfer excited states. Several approaches to the evaluation of the outer-shell barrier are explored in an investigation of the distance dependence of the nuclear factor in intramolecular electron-transfer processes. 39 refs., 14 figs., 3 tabs.

Sutin, N.; Brunschwig, B.S.; Creutz, C.; Winkler, J.R.

1988-01-01

93

Excitation transfer, charge transfer and hydrogen abstraction reaction, kinetics studies of modification of DNA components  

International Nuclear Information System (INIS)

Laser induced acetone-sensitized excitation of DNA components offered, for the first time, T-T absorption spectra and direct triplet states kinetics of cytosine, cytidine, and dCMP, adenine, adenosine and dAMP and guanosine. Mechanisms and kinetics of electron transfer reactions between triplet acetone and purine bases and triplet thymine and electroaffinic modifiers have been elucidated. Rapid electron transfer from hydroxycinnamic acid derivatives to oxidizing OH adducts of pyrimidines have been performed. Fast reduction and repair of oxidizing damage of pyrimidines have been achieved accordingly. (author)

94

Electron transfer: Lower tunnel barriers  

Science.gov (United States)

A better understanding of electron transfer through molecules could provide the basis for many technological breakthroughs. Now, the rate of electron transfer has been enhanced in a family of molecules by making them more rigid, and this phenomenon may be explained by the loss of electronic energy to vibrations.

Miller, John R.

2014-10-01

95

Photoinduced electron transfer in a tris(2,2`-bipyridine)-C{sub 60}-ruthenium(II) dyad: evidence of charge recombination to a fullerene excited state  

Energy Technology Data Exchange (ETDEWEB)

A novel fulleropyrrolidine covalently linked to a tris(2,2`-bipyridine)-ruthenium(II) complex is presented. While electrochemical investigations suggest the absence of any ground-state interaction between ruthenium and fullerence chromophores, photoinduced optical absorption studies clearly show that electron transfer occurs to afford the Ru{sup 3+}-C{sub 60}{sup .-} pair with characteristic fullerene radical anion band at {lambda}{sub max}=1040 nm. (orig.)

Maggini, M.; Mondini, S.; Scorrano, G. [Centro Meccanismi Reazioni Organiche-CNR, Padova (Italy). Dipt. di Chimica Organica; Guldi, D.M. [Notre Dame Univ., IN (United States). Radiation Lab.; Paolucci, F.; Ceroni, P.; Roffia, S. [Bologna Univ. (Italy). Dipt. di Chimica G. Ciamician

1998-10-01

96

Photoreduction of indigo dyes by electron donors. One- and two-electron-transfer reactions as a consequence of excited-state quenching  

Energy Technology Data Exchange (ETDEWEB)

The indigoid dyes, thioindigo (TI), N,N'-diacetylindigo, (NDI) and oxalylindigo (OI), all undergo reduction upon irradiation of the dyes in the presence of electron donors such as triethylamine (TEA) or N-benzyl-1,4-dihydronicotinamide (BNAH). Product analysis by NMR and high-resolution mass spectrometry has shown that the products for TI and NDI are the formal H/sub 2/ adducts TIH/sub 2/ and NDIH/sub 2/; the product for OI has been shown to be the semireduced radical OIH which is readily detected by its characteristic ESR spectrum. Mechanistic studies have been carried out for the visible-light-induced reduction of the three dyes.

Schanze, K.S.; Lee, L.Y.C.; Giannotti, C.; Whitten, D.G.

1986-05-14

97

Photochemistry on surfaces. Intermolecular energy electron transfer processes between excited Ru(bpy){sub 3}{sup 2+} and H-aggregates of cresyl violet on SiO{sub 2} and SnO{sub 2} colloids  

Energy Technology Data Exchange (ETDEWEB)

Cresyl violet, a cationic dye (CV{sup +}), forms H-aggregates on the negatively charged SiO{sub 2} and SnO{sub 2} colloids. These aggregates exhibit broad absorbance around 520 nm. By coadsorbing a sensitizer, Ru(bpy){sub 3}{sup 2+}, we are able to characterize the triplet excited state and reduced form of dye-aggregates on the colloidal SiO{sub 2} and SnO{sub 2} suspensions. On SiO{sub 2} surfaces, the excited state quenching of Ru(bpy){sub 3}{sup 2+} by dye-aggregates proceeds via an energy transfer mechanism. Picosecond laser flash photolysis experiments indicate that such a surface-promoted energy transfer is completed within 20 ps. On the other hand dye-aggregates adsorbed onto SnO{sub 2} colloids undergo photosensitized reduction since the excited sensitizer, Ru(bpy){sub 3} {sup 2+}, is efficiently quenched by the semiconductor support. The role of support material in promoting energy and electron transfer processes is described. 87 refs., 11 figs.

Liu, D.; Hug, G.L.; Kamat, P.V. [Univ. of Notre Dame, IN (United States)

1995-11-09

98

Electron beam excitation of surface plasmon polaritons.  

Science.gov (United States)

In this paper, the excitations of surface plasmon polaritons (SPPs) by both perpendicular and parallel electron beam are investigated. The results of analytical theory and numerical calculation show that the mechanisms of these two excitations are essentially different, and the behavior and properties of SPPs in metal structures strongly depend on the methods of excitation. For the perpendicular excitation, SPPs contain plenty of frequency components, propagate with attenuation and are always accompanied with the transition radiation. Whereas for parallel excitation, SPPs waves are coherent, tunable, propagating without attenuation and the transition radiation does not occur. We also show that there are two modes for the parallel excited SPPs on the metal films and they all can be excited efficiently by the parallel moving electron beam. And the operating frequency of SPPs can be tuned in a large frequency range by adjusting the beam energy. PMID:25321010

Gong, Sen; Hu, Min; Zhong, Renbin; Chen, Xiaoxing; Zhang, Ping; Zhao, Tao; Liu, Shenggang

2014-08-11

99

Theoretical analysis of excited states and energy transfer mechanism in conjugated dendrimers.  

Science.gov (United States)

The excited states of the phenylene ethynylene dendrimer are investigated comprehensively by various electronic-structure methods. Several computational methods, including SCS-ADC(2), TDHF, TDDFT with different functionals (B3LYP, BH&HLYP, CAM-B3LYP), and DFT/MRCI, are applied in systematic calculations. The theoretical approach based on the one-electron transition density matrix is used to understand the electronic characters of excited states, particularly the contributions of local excitations and charge-transfer excitations within all interacting conjugated branches. Furthermore, the potential energy curves of low-lying electronic states as the functions of ethynylene bonds are constructed at different theoretical levels. This work provides us theoretical insights on the intramolecular excited-state energy transfer mechanism of the dendrimers at the state-of-the-art electronic-structure theories. © 2014 Wiley Periodicals, Inc. PMID:25413342

Huang, Jing; Du, Likai; Hu, Deping; Lan, Zhenggang

2015-01-30

100

Neutron scattering investigation of magnetic excitations at high energy transfers  

International Nuclear Information System (INIS)

With the advance of pulsed spallation neutron sources, neutron scattering investigation of elementary excitations in magnetic materials can now be extended to energies up to several hundreds of MeV. We have measured, using chopper spectrometers and time-of-flight techniques, the magnetic response functions of a series of d and f transition metals and compounds over a wide range of energy and momentum transfer. In PrO2, UO2, BaPrO3 and CeB6 we observed crystal-field transitions between the magnetic ground state and the excited levels in the energy range from 40 to 260 MeV. In materials exhibiting spin-fluctuation or mixed-valent character such as Ce 74Th 26, on the other hand, no sharp crystal-field lines but a broadened quasielastic magnetic peak was observed. The line width of the quasielastic component is thought to be connected to the spin-fluctuation energy of the 4f electrons. The significance of the neutron scattering results in relation to the ground state level structure of the magnetic ions and the spin-dynamics of the f electrons is discussed. Recently, in a study of the spin-wave excitations in itinerant magnetic systems, we have extended the spin-wave measurements in ferromagnetic iron up to about 160 MeV. Neutron scattering data at high energy transfers are of particular interest because they provide direct comparison with recent theories of itinerant magnetism. 26 references, 7 figures

101

Lifetime of electronic excitations in metal nanoparticles  

Energy Technology Data Exchange (ETDEWEB)

Electronic excitations in metal particles with sizes up to a few nanometers are shown to have a one-electron character when a laser pulse is applied off the plasmon resonance. The calculated lifetimes of these excitations are in the femtosecond timescale but their values are substantially different from those in bulk. This deviation can be explained from the large weight of the excitation wave function in the nanoparticle surface region, where dynamic screening is significantly reduced. The well-known quadratic dependence of the lifetime with the excitation energy in bulk breaks down in these finite-size systems.

Quijada, M; Diez Muino, R; Echenique, P M [Centro de Fisica de Materiales CFM-Materials Physics Center MPC, Centro Mixto CSIC-UPV/EHU, Edificio Korta, Avenida de Tolosa 72, 20018 San Sebastian (Spain); Borisov, A G; Alonso, J A, E-mail: rdm@ehu.e [Donostia International Physics Center DIPC, P Manuel de Lardizabal 4, 20018 San Sebastian (Spain)

2010-05-15

102

Ultrafast electronic relaxation of excited state vitamin B{sub 12} in the gas phase  

Energy Technology Data Exchange (ETDEWEB)

The time evolution of electronically excited vitamin B{sub 12} (cyanocobalamin) has been observed for the first time in the gas phase. It reveals an ultrafast decay to a state corresponding to metal excitation. This decay is interpreted as resulting from a ring to metal electron transfer. This opens the observation of the excited state of other complex biomimetic systems in the gas phase, the key to the characterisation of their complex evolution through excited electronic states.

Shafizadeh, Niloufar [Laboratoire de Photophysique Moleculaire, U.P.R. 3361 CNRS Bat 210, Universite de Paris-Sud, 91405 Orsay, Cedex (France)], E-mail: Niloufar.Shafizadeh@u-psud.fr; Poisson, Lionel; Soep, Benoit [Laboratoire Francis Perrin, CEA/DSM/DRECAM/SPAM - CNRS URA 2453, CEA Saclay, 91191 Gif-sur-Yvette Cedex (France)

2008-06-23

103

Symmetry characterization of electrons and lattice excitations  

Directory of Open Access Journals (Sweden)

Full Text Available Symmetry concerns all aspects of a physical system from the electronic orbitals to structural and magnetic excitations. In this article we will try to elaborate the fundamental connection between symmetry and excitations. As excitations are manyfold in physical systems it is impossible to treat them exhaustively. We thus concentrate on the two topics of Bloch electrons and phonons. These two examples are complementary in the sense that Bloch electrons describe single particles in an external periodic potential while phonons exemplify a decoupled system of interacting particles. The way we develop the argument gives as by-product a short account of molecular orbitals and molecular vibrations.

Schober H.

2012-03-01

104

Hot-electron transfer from semiconductor nanocrystals.  

Science.gov (United States)

In typical semiconductor solar cells, photons with energies above the semiconductor bandgap generate hot charge carriers that quickly cool before all of their energy can be captured, a process that limits device efficiency. Although fabricating the semiconductor in a nanocrystalline morphology can slow this cooling, the transfer of hot carriers to electron and hole acceptors has not yet been thoroughly demonstrated. We used time-resolved optical second harmonic generation to observe hot-electron transfer from colloidal lead selenide (PbSe) nanocrystals to a titanium dioxide (TiO2) electron acceptor. With appropriate chemical treatment of the nanocrystal surface, this transfer occurred much faster than expected. Moreover, the electric field resulting from sub-50-femtosecond charge separation across the PbSe-TiO2 interface excited coherent vibrations of the TiO2 surface atoms, whose motions could be followed in real time. PMID:20558714

Tisdale, William A; Williams, Kenrick J; Timp, Brooke A; Norris, David J; Aydil, Eray S; Zhu, X-Y

2010-06-18

105

Femtosecond excitation transfer processes in biliprotein trimers  

Science.gov (United States)

Femtosecond processes in allophycocyanin, C-phycocyanin and phycoerythrocyanin trimers and monomers have been examined by means of polarization pump-probe technique. No femtosecond kinetics were observed in monomeric preparations. The isotropic absorption recovery kinetics with (tau) equals 440 +/- 50 fs which is not accompanied by anisotropy decay kinetics was obtained in allophycocyanin trimers at 612 nm. The conclusion about energy transfer between neighboring (alpha) 84 and (beta) 84 chromophores with different absorption spectra was made. The proposed model takes into account a stabilizing role of the linker peptide. Spectral and kinetic measurements were made in the 635 - 690 nm spectral region where the proposed acceptor should absorb. The bleaching of the 650-nm band occurs with a delay relative to the bleaching at 615 nm. Only a rise term was observed at 658 nm in consistence with the proposed model. Anisotropy values calculated around 650 nm at 3 ps after excitation are in the range 0.1 - 0.25 corresponding to an angle of 30 degree(s) - 45 degree(s) between the donor and acceptor transition dipole moments. A 500-fs absorption recovery and anisotropy decay process was obtained for C-phycocyanin trimers and explained by Forster energy transfer over 20.8 angstroms between neighboring (alpha) 84 and (beta) 84 chromophores of different monomeric subunits having similar absorption spectra and with a 65 degree(s) angle between their orientations. Energy transfer between violobilin ((alpha) 84) and phycocyanobilin ((beta) 84) chromophores was examined in donor and acceptor spectral regions of phycoerythrocyanin trimers, and was found to take 400 fs.

Sharkov, A. V.; Khoroshilov, E. V.; Kryukov, I. V.; Palsson, Lars-Olof; Kryukov, P. G.; Fischer, R.; Scheer, Hella-Christin; Gillbro, Tomas

1993-06-01

106

Medium effects in photoinduced electron transfer reactions  

International Nuclear Information System (INIS)

The transfer of an electron between two molecules is a fundamental chemical process of great significance in biochemistry as well as in general chemistry. Electron transfer reactions can be induced by the absorption of light - as in photosynthesis - so that one of the molecules reacts through an electronically excited state; a net storage of chemical energy may then take place. When electron transfer involves molecules in condensed phase, the role of the liquid or solid medium must be considered. In the first place, a polar solvent may promote electron transfer through the stabilization of ion pairs and the separation of ions; but if the polar solvent must reorganize prior to the reaction, then an activation barrier will exist against electron transfer in polar solvents. This article gives a citical review of this field, with some novel ideas concerning the role of the medium (liquid solvent or solid matrix) in the overall energy balance of electron transfer, in the kinetics of the reaction, and in the further process of charge separation. It is suggested in particular that no dielectric screening by a polar solvent can exist when ions are formed in direct contact; and that the extensive reorganization of the solvent prior to electron transfer is so unlikely as to be discounted. In these respects the model presented in this article diverges from the commonly accepted model of Weller and of Marcus and Hush. In the Conclusion section some areas of particular importance ction some areas of particular importance for further research in this field are outlined. (author) 28 refs., 20 figs., 2 tabs

107

Resonant electron transfer between quantum dots  

OpenAIRE

An interaction of electromagnetic field with a nanostructure composed of two quantum dots is studied theoretically. An effect of a resonant electron transfer between the localized low-lying states of quantum dots is predicted. A necessary condition for such an effect is the existence of an excited bound state whose energy lies close to the top of the barrier separating the quantum dots. This effect may be used to realize the reversible quantum logic gate NOT if the superposi...

Openov, Leonid A.

1999-01-01

108

Electron-impact vibrational excitation of tetrahydrofuran  

Science.gov (United States)

Low-energy differential and integral cross sections for the electron-impact vibrational excitation of tetrahydrofuran are presented. The data concern four features in the vibrational excitation electron energy-loss spectrum of tetrahydrofuran over the energy-loss range of 0-0.9 eV. The results show weak influence from long-range dipole interactions, being mainly isotropic. Comparison with earlier work is presented.

Khakoo, M. A.; Orton, D.; Hargreaves, L. R.; Meyer, N.

2013-07-01

109

Electron-Impact Excitation of Tellurium Vapor  

Science.gov (United States)

Excitation processes of tellurium vapor in collisions with low-energy electrons (0-50 eV) are studied. Emission spectra in the 200-525 nm range produced by electrons with energies of 10, 15, 20, and 30 eV are taken, and the optical excitation functions of the most intense atomic lines and molecular bands are measured. It is found that at temperatures of 270-320°C the vapor contains a large amount of diatomic Te2 molecules, as well as polyatomic tellurium molecules, while atomic Te* lines are formed by dissociative excitation of tellurium molecules.

Shpenik, O. B.; Erdevdy, N. M.; Zvenighorodsky, V. V.; Markush, P. P.

2014-09-01

110

Collective Excitations of Dirac Electrons in Graphene  

CERN Document Server

Two-dimensional electrons in graphene are known to behave as massless fermions with Dirac-Weyl type linear dispersion near the Dirac crossing points. We have investigated the collective excitations of this system in the presence or absence of an external magnetic field. Unlike in the conventional two-dimensional electron system, the $\

Apalkov, V; Chakraborty, T; Apalkov, Vadim; Wang, Xue-Feng; Chakraborty, Tapash

2006-01-01

111

Bioinspired electron-transfer systems and applications  

International Nuclear Information System (INIS)

Bioinspired electron-transfer systems including artificial photosynthesis and respiration are presented herein together with some of their applications. First, multi-step electron-transfer systems composed of electron donor-acceptor ensembles have been developed, mimicking functions of the photosynthetic reaction center. However, a significant amount of energy is lost during the multi-step electron-transfer processes. Then, as an alternative to conventional charge-separation functional molecular models based on multi-step long-range electron transfer within redox cascades, simple donor-acceptor dyads have been developed to attain along-lived and high energy charge-separated state without significant loss of excitation energy, by fine control of the redox potentials and of the geometry of donor-acceptor dyads that have small reorganization energies of electron transfer. Such simple molecular dyads, capable of fast charge separation but extremely slow charge recombination, have significant advantages with regard to synthetic feasibility, providing a variety of applications including construction of organic solar cells and development of efficient photo-catalytic systems for the solar energy conversion. An efficient four-electron reduction of dioxygen to water by one-electron reductants such as ferrocene derivatives as well as by an NADH analog has also been achieved as a respiration model by using a cofacial dicobalt porphyrin that can form the ?-peroxo Co(III)-O2hat can form the ?-peroxo Co(III)-O2Co(III) complex. The catalytic mechanism of the four-electron reduction of dioxygen has been clarified based on the detailed kinetic study and the detection of the intermediate. (author)

112

Photoinduced energy and electron transfer in rubrene-benzoquinone and rubrene-porphyrin systems  

Science.gov (United States)

Excited-state electron and energy transfer from singlet excited rubrene (Ru) to benzoquinone (BQ) and tetra-(4-aminophenyl) porphyrin (TAPP) were investigated by steady-state absorption and emission, time-resolved transient absorption, and femtosecond (fs)-nanosecond (ns) fluorescence spectroscopy. The low reduction potential of BQ provides the high probability of electron transfer from the excited Ru to BQ. Steady-state and time-resolved results confirm such an excited electron transfer scenario. On the other hand, strong spectral overlap between the emission of Ru and absorption of TAPP suggests that energy transfer is a possible deactivation pathway of the Ru excited state.

Khan, Jafar I.; Abbas, Abdullah Saud; Aly, Shawkat M.; Usman, Anwar; Melnikov, Vasily A.; Alarousu, Erkki; Mohammed, Omar F.

2014-11-01

113

Electronic excitation of helium and krypton by electron impact  

International Nuclear Information System (INIS)

A crossed-beam electron impact spectrometer is modified to enable the study of near-threshold electronic excitation of He, and Kr by electron impact. Absolute differential cross sections for excitation of the individual n = 2 states of He and 4p55s states of Kr are measured within the first four electron volts of threshold. In He, pronounced angle-dependent structures are apparent in the threshold excitation of the 23P state and in the near-threshold excitation of the 23S state. These experimental observations, when viewed in light of recent theoretical considerations, suggest the existence of a new 2P resonance near 20.8 eV. The results on Kr demonstrate that the angular momentum of the Kr+ ion core (4p5) has a strong influence on the excitation cross sections, both on the relative magnitudes of the cross sections and on the relative strengths of resonant structures in the excitation functions. The angular momentum of the entire electron cloud of an excited state has less of an effect on the cross sections. It manifests itself primarily in the relative magnitudes of the two metastable cross sections. These observations emphasize the weakness of the coupling between the core and the outer electrons in the electron-Kr system

114

Electron-impact excitation of atomic copper  

International Nuclear Information System (INIS)

Cross sections are calculated for excitation of Cu for elastic scattering of electrons from the 4s ground state and for excitation to excited states 3d94s2, 4p, and 4d. A three-state close-coupling approximation is used for the energy range 3.8 < E < 10 eV, and a four-state one for 6 < E < 100 eV. Accurate target functions are used in the expansion. Poor agreement is obtained with renormalized experimental data of Trajmar et al

115

Electron-impact excitation of oxygenlike krypton  

International Nuclear Information System (INIS)

Electron-impact excitation cross sections have been calculated for transitions from the ground state 2s22p43P to the n = 2 and n = 3 excited states of oxygenlike krypton. Configuration-interaction-type wave functions were employed, and the cross sections were calculated in a close-coupling approximation for impact energies ranging from near threshold to 10 keV. Coupling between the n = 2 and n = 3 excitation levels was generally found to be weak, and a two-state close-coupling approximation should be adequate for calculating reliable cross sections for most transitions

116

Computer simulation of cluster impact induced electronic excitation of solids  

Science.gov (United States)

We present a computational study of electronic excitation upon bombardment of a metal surface with cluster projectiles. Our model employs a molecular dynamics (MD) simulation to calculate the particle dynamics following the projectile impact. Kinetic excitation is implemented via two mechanisms describing the electronic energy loss of moving particles: autoionization in close binary collisions and a velocity proportional friction force resulting from direct atom-electron collisions. Two different friction models are compared with respect to the predicted sputter yields after single atom and cluster bombardment. We find that a density dependent friction coefficient leads to a significant reduction of the total energy transferred to the electronic sub-system as compared to the Lindhard friction model, thereby strongly enhancing the predicted sputter yield under cluster bombardment conditions. In contrast, the yield predicted for monoatomic projectile bombardment remains practically unchanged.

Weidtmann, B.; Hanke, S.; Duvenbeck, A.; Wucher, A.

2013-05-01

117

Computer simulation of cluster impact induced electronic excitation of solids  

Energy Technology Data Exchange (ETDEWEB)

We present a computational study of electronic excitation upon bombardment of a metal surface with cluster projectiles. Our model employs a molecular dynamics (MD) simulation to calculate the particle dynamics following the projectile impact. Kinetic excitation is implemented via two mechanisms describing the electronic energy loss of moving particles: autoionization in close binary collisions and a velocity proportional friction force resulting from direct atom–electron collisions. Two different friction models are compared with respect to the predicted sputter yields after single atom and cluster bombardment. We find that a density dependent friction coefficient leads to a significant reduction of the total energy transferred to the electronic sub-system as compared to the Lindhard friction model, thereby strongly enhancing the predicted sputter yield under cluster bombardment conditions. In contrast, the yield predicted for monoatomic projectile bombardment remains practically unchanged.

Weidtmann, B.; Hanke, S.; Duvenbeck, A. [Fakultät für Physik, Universität Duisburg-Essen, 47048 Duisburg (Germany); Wucher, A., E-mail: andreas.wucher@uni-deu.de [Fakultät für Physik, Universität Duisburg-Essen, 47048 Duisburg (Germany)

2013-05-15

118

Photoinduced electron transfer from a fundamental understanding to potential applications  

OpenAIRE

Although the transfer of an electron from a donor to an acceptor after one of them has been electronically excited by the absorption of light appears to be a very simple reaction, there are still many open questions concerning the detailed mechanism of photoinduced electron transfer. It is nevertheless a fascinating and very important reaction, given its key importance in photosynthesis and many modern approaches to solar energy conversion. The objective of this PhD thesis was thus to use ult...

Banerji, Nathalie Renuka

2009-01-01

119

Neutral excitations in quasi-1D strongly correlated electron systems  

International Nuclear Information System (INIS)

Novel neutral spin-0 excitation for quasi-1D systems with one electron per site are obtained within a localized scheme. Our ground state wave-function is defined by a variational localized - site cluster - expanded ansatz and these excitations are naturally built upon it. All computations are easily carried out by a transfer matrix technique that enables explicit inclusion of electron - correlation effects and, at the same time, provides a quasi - particle band - theoretic picture of these excitations. This treatment is applied, in particular, to a polyphene strip. At k=0 the energy gap corresponds to a dipole-allowed transition in the visible region, our result compares fairly well with observed decreasing transitions for finite polyphene strips. (author). 14 refs, 1 fig

120

Excitations and benchmark ensemble density functional theory for two electrons  

International Nuclear Information System (INIS)

A new method for extracting ensemble Kohn-Sham potentials from accurate excited state densities is applied to a variety of two-electron systems, exploring the behavior of exact ensemble density functional theory. The issue of separating the Hartree energy and the choice of degenerate eigenstates is explored. A new approximation, spin eigenstate Hartree-exchange, is derived. Exact conditions that are proven include the signs of the correlation energy components and the asymptotic behavior of the potential for small weights of the excited states. Many energy components are given as a function of the weights for two electrons in a one-dimensional flat box, in a box with a large barrier to create charge transfer excitations, in a three-dimensional harmonic well (Hooke's atom), and for the He atom singlet-triplet ensemble, singlet-triplet-singlet ensemble, and triplet bi-ensemble

121

Coherence in electron-impact excitation of helium  

International Nuclear Information System (INIS)

This thesis describes an experimental study into the electron-impact excitation to the 33P, 31D and 33D states of Helium. The scattered electron and the photon, emitted by the excited atom, are measured in coincidence. The parameters, which can be varied, are the scattering angle and the kinetic energy of the projectile. Two parameters, which are used to characterize the excited state, are the angular momentum transferred to the atom, L perpendicular, and the alignment angle ?. It is shown that results of measurements on 31D excitation with photon detection perpendicular to the scattering plane do not agree in the small scattering angle region with any of the model calculations currently available. Remarkable is the sign of L perpendicular, which appears to start of negatively at 60 eV. It is shown that for 33P excitation the predicted large value of ? is indeed found experimentally. This supports the suggestion that exchange scattering is underestimated in model calculations for 1P excitation. Another result is that for 1P and 3P excitation the behaviour of L perpendicular as a function of the scattering angle can be related at different impact energies with the help of a partial wave expansion. A scaling relation can be formulated for the behaviour of L perpendicular. The influence of a negative ion resonance to excitation of the 33D state is investigated. Both in coincidence and non-coincidence measurements the presence of the resonance yields information on both the direct and indirect excitation of the 33D state. It is shown that the coincident measurement gives an unique opportunity to determine the excited 33D state completely. Results of measurements with photon detection in the scattering plane are given. They supplement previous 31D and 33D results and allow physical parameters, such as L perpendicular and ?, to be obtained. (H.W.). 132 refs.; 20 figs.; 18 tabs

122

Ultrafast Electron Dynamics Theory of Photo-excited Ruthenium Complexes  

OpenAIRE

An explanation is provided for the ultrafast photo-excited electron dynamics in low-spin Ruthenium (II) organic complexes. The experimentally-observed singlet to triplet decay in the metal-to-ligand charge-transfer (MLCT) states contradicts the expectation that the system should oscillate between the singlet and triplet states in the presence of a large spin-orbit coupling and the absence of a significance change in metal-ligand bond length. This dilemma is solved with a nov...

Chang, Jun; Fedro, A. J.; Veenendaal, Michel

2010-01-01

123

Dissociation mechanisms of excited CH3X (X = Cl, Br, and I) formed via high-energy electron transfer using alkali metal targets  

Science.gov (United States)

High-energy electron transfer dissociation (HE-ETD) on collisions with alkali metal targets (Cs, K, and Na) was investigated for CH3X+ (X = Cl, Br, and I) ions by a charge inversion mass spectrometry. Relative peak intensities of the negative ions formed via HE-ETD strongly depend on the precursor ions and the target alkali metals. The dependency is explained by the exothermicities of the respective dissociation processes. Peak shapes of the negative ions, especially of the X- ions, which comprise a triangle and a trapezoid, also strongly depend on the precursor ions and the target alkali metals. The trapezoidal part of the I- peak observed with the Na target is more dominant and much broader than that with the Cs target. This dependence on the targets shows an inverse relation between the peak width and the available energy, which corresponds to the exothermicity assuming formation of fragment pair in their ground internal states. From a comparison of the kinetic energy release value calculated from the trapezoidal shape of I- with the available energy of the near-resonant level on the CH3I potential energy curve reported by ab initio calculations, the trapezoidal part is attributed to the dissociation to CH3 + I(2P3/2) via the repulsive 3Q1 state of CH3I, which is not dominant in the photo-dissociation of CH3I. The observation of trapezoid shape of the CH2I- peak with the Cs target indicates spontaneous dissociation via repulsive potential from the 3R2 Rydberg state, although the correlation between the 3R2 Rydberg state and relevant repulsive states has not been reported by any theoretical calculation.

Hayakawa, Shigeo; Tsujinaka, Taiga; Fujihara, Akimasa

2012-11-01

124

Vibronic effects on the low-lying electronic excitations in CO2 induced by electron impact.  

Science.gov (United States)

We report an angle-resolved electron energy loss spectroscopy (EELS) study on the valence-shell electronic excitations of CO2. Experimentally, momentum-transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles for low-lying electronic excitations have been derived from EELS spectra measured at incident electron energy of 3 keV. Theoretically, we have calculated GOS profiles using wave functions at the equation-of-motion coupled-cluster singles and doubles level. In the calculation, vibronic effects are taken into account by evaluating the electronic transition amplitudes along the individual normal coordinates. The calculation satisfactorily reproduces the experiment for the (1)?g and (1)?u transitions and reveals prominent roles of strong coupling between the associated mixed-Rydberg-valence and valence excited states through the bending vibration of CO2. Vibronic effects on GOS profiles have also been examined for the (1)?u (+), (1)?u, and 2(1)?g transitions. PMID:23676048

Watanabe, Noboru; Hirayama, Tsukasa; Suzuki, Daisuke; Takahashi, Masahiko

2013-05-14

125

Core excitation of Li by electron impact  

International Nuclear Information System (INIS)

Cross sections for the excitation of a core electron, which leads to autoionization, in lithium (Li) atomic system by electron impact have been calculated with use of the single-configuration Hartree-Fock wave function within the asymptotic Green's-function approximation (AGFA) in the low-bombarding-energy region. Comparison is made with available results. Our investigation demonstrates that the AGFA supports the R-matrix as well as the distorted-wave Born-approximation behavior

126

Electroluminescence from graphene excited by electron tunneling.  

Science.gov (United States)

We use low-energy electron tunneling to excite electroluminescence in single layer graphene. Electrons are injected locally using a scanning tunneling microscope and the luminescence is analyzed using a wide-angle optical imaging system. The luminescence can be switched on and off by inverting the tip-sample bias voltage. The observed luminescence is explained in terms of a hot luminescence mechanism. PMID:24407020

Beams, Ryan; Bharadwaj, Palash; Novotny, Lukas

2014-02-01

127

Electronic excitation and ion source optimization  

International Nuclear Information System (INIS)

The electronic excitation cross sections leading to H2(v'') molecules are discussed. The effect of shortening the length of the first chamber of a tandem configuration for the purpose of reducing the atomic concentration is shown to enhance the extracted current density

128

Simultaneous electron capture and excitation in ion-atom collisions  

International Nuclear Information System (INIS)

A review of recent efforts to observe simultaneous electron capture-and-K-shell excitation in ion-atom collisions is presented. This process which has been referred to as resonant-transfer-and-excitation (RTE), is qualitatively analogous to dielectronic recombination (inverse Auger transition) in free-electron-ion collisions, and, hence, is expected to be resonant. Experimentally, events having the correct signature for simultaneous capture and excitation are isolated by detecting projectile K x rays in coincidence with ions which capture a single electron. In a recent experiment involving 70-160 MeV S13+ ions incident on Ar, a maximum was observed in the yield of projectile K x rays associated with electron capture. This maximum is attributed to simultaneous capture-and excitation. The position (120 MeV) and width (60 MeV) of the observed maximum are in good agreement with theoretical calculations. The data indicate that RTE is an important mechanism for inner-shell vacancy production in the energy range studied

129

75 FR 31665 - Electronic Fund Transfers  

Science.gov (United States)

...implements the Electronic Fund Transfer...that overdraw a consumer's account...implements the Electronic Fund Transfer...without the consumer's affirmative...CFR Part 205 Consumer protection, Electronic fund...

2010-06-04

130

75 FR 9120 - Electronic Fund Transfers  

Science.gov (United States)

...implements the Electronic Fund Transfer...that overdraw a consumer's account...implements the Electronic Fund Transfer...without the consumer's affirmative...CFR Part 205 Consumer protection, Electronic fund...

2010-03-01

131

Plasmonic coupling and long-range transfer of an excitation along a DNA nanowire.  

Science.gov (United States)

We demonstrate an excitation transfer along a fluorescently labeled dsDNA nanowire over a length of several micrometers. Launching of the excitation is done by exciting a localized surface plasmon mode of a 40 nm silver nanoparticle by 800 nm femtosecond laser pulses via two-photon absorption. The plasmonic mode is subsequently coupled or transformed to excitation in the nanowire in contact with the particle and propagated along it, inducing bleaching of the dyes on its way. In situ as well as ex situ fluorescence microscopy is utilized to observe the phenomenon. In addition, transfer of the excitation along the nanowire to another nanoparticle over a separation of 5.7 ?m was clearly observed. The nature of the excitation coupling and transfer could not be fully resolved here, but injection of an electron into the DNA from the excited nanoparticle and subsequent coupled transfer of charge (Dexter) and delocalized exciton (Frenkel) is the most probable mechanism. However, a direct plasmonic or optical coupling and energy transfer along the nanowire cannot be totally ruled out either. By further studies the observed phenomenon could be utilized in novel molecular systems, providing a long-needed communication method between molecular devices. PMID:23305550

Toppari, J Jussi; Wirth, Janina; Garwe, Frank; Stranik, Ondrej; Csaki, Andrea; Bergmann, Joachim; Paa, Wolfgang; Fritzsche, Wolfgang

2013-02-26

132

Electronic excitation of methane molecule by electron impact  

International Nuclear Information System (INIS)

In this work, it is presented elastic and inelastic differential cross sections for the electronic excitation of the Triplet (3T2) of CH4 molecule calculated for electron incident energies of 20 and 30 eV. It is employed the Schwinger multichannel method and these results are compared to the ones available in the literature. (A.C.A.S.)

133

Transient negative photoconductance in a charge transfer double quantum well under optical intersubband excitation  

Science.gov (United States)

Recently, it was shown that an electron-hole radiative recombination is induced by a mid-infrared light exciting an intersubband transition in a charge transfer double quantum well (CTDQW). This recombination was attributed to an upstream transfer of electrons from an electron-rich well to a hole-rich well. In this study, we investigated the electrical response of a CTDQW under intersubband optical excitation, and found that a positive photocurrent, opposite in sign and proportional to the applied electric field, accompanies the intersubband-transition-induced luminescence (ITIL) signal. A negative photocurrent component was also observed and attributed to heating processes. This work brings a further evidence of the ITIL process and shows that an important proportion of the carriers are consumed by the transfer of electrons.

Rüfenacht, M.; Tsujino, S.; Sakaki, H.

1998-06-01

134

Vibronic speed-up of the excitation energy transfer in the Fenna-Matthews-Olson complex  

CERN Document Server

We show that the efficient excitation energy transfer in the Fenna-Matthews-Olson molecular aggregate under realistic physiological conditions is fueled by underdamped vibrations of the embedding proteins. For this, we present numerically exact results for the quantum dynamics of the excitons in the presence of nonadiabatic vibrational states in the Fenna-Matthews-Olson aggregate employing a environmental fluctuation spectral function derived from experiments. Assuming the prominent 180 cm$^{-1}$ vibrational mode to be underdamped, we observe, on the one hand, besides vibrational coherent oscillations between different excitation levels of the vibration also prolonged electronic coherent oscillations between the initially excited site and its neighbours. On the other hand, however, the underdamped vibrations provide additional channels for the excitation energy transfer and by this increase the transfer speed by up to $30\\%$ .

Nalbach, P; Thorwart, M

2013-01-01

135

L-shell alignment observed after resonant transfer and excitation in U90+ ? C collisions  

Science.gov (United States)

With heavy, highly-charged ions the electron-electron correlation is probed in the relativistic domain by Resonant Transfer and Excitation, RTE. For the studied case of initially He-like U90+ ions the KLL-RTE splits into three resonance groups according to the j values of the electrons involved in the doubly-excited intermediate states. For a K to L excitation into a j = ½ state with a simultaneous capture also to a j' = ½ state, relativistic effects yield a strong cross section enhancement. The doubly excited states with jj' = 1/2-3/2 show a strong anistropy for the radiative ground state transition of the j' = 3/2 electron manifesting a pronounced non-isotropic population of the magnetic substates.

Mokler, P. H.; Kandler, T.; Stöhlker, Th; Geissel, H.; Kozhuharov, C.; Rymuza, P.; Scheidenberger, C.; Stachura, Z.; Warczak, A.; Dunford, R. W.

1997-01-01

136

Photoinitiated electron transfer in multi-chromophoric species: Synthetic tetrads and pentads  

Energy Technology Data Exchange (ETDEWEB)

This research project involves the design, synthesis and study of the molecules which mimic many of the important aspects of photosynthetic electron and energy transfer. Specifically, the molecules are designed to mimic the following aspects of natural photosynthetic multistep electron transfer: electron donation from a tetrapyrrole excited singlet state, electron transfer between tetrapyrroles, electron transfer from tetrapyrroles to quinones, and electron transfer between quinones with different redox properties. In addition, they model carotenoid antenna function in photosynthesis (singlet-singlet energy transfer from carotenoid polyenes to chlorophyll) and carotenoid photoprotection from singlet oxygen damage (triplet-triplet energy transfer from chlorophyll to carotenoids).

1990-02-14

137

Erbium emission in MOS light emitting devices: from energy transfer to direct impact excitation  

International Nuclear Information System (INIS)

The electroluminescence (EL) at 1.54 µm of metal–oxide–semiconductor (MOS) devices with Er3+ ions embedded in the silicon-rich silicon oxide (SRSO) layer has been investigated under different polarization conditions and compared with that of erbium doped SiO2 layers. EL time-resolved measurements allowed us to distinguish between two different excitation mechanisms responsible for the Er3+ emission under an alternate pulsed voltage signal (APV). Energy transfer from silicon nanoclusters (Si-ncs) to Er3+ is clearly observed at low-field APV excitation. We demonstrate that sequential electron and hole injection at the edges of the pulses creates excited states in Si-ncs which upon recombination transfer their energy to Er3+ ions. On the contrary, direct impact excitation of Er3+ by hot injected carriers starts at the Fowler–Nordheim injection threshold (above 5 MV cm?1) and dominates for high-field APV excitation. (paper)

138

Electron spectroscopy of collisional excited atoms  

International Nuclear Information System (INIS)

In this thesis measurements are described in which coincidences are detected between scattered projectiles and emitted electrons. This yields information on two-electron excitation processes. In order to show what can be learnt from coincidence experiments a detailed theoretical analysis is given. The transition amplitudes, which contain all the information, are introduced (ch.2). In ch.3 the experimental set-up is shown. The results for the Li+-He system are shown in ch. 7 and are compared with predictions based on the Molecular-Orbitalmodel which however does not account for two-excitation mechanisms. With the transition amplitudes also the wave function of the excited atom has been completely determined. In ch.8 the shape of the electron cloud, induced by the collision, is derived from the amplitudes. The relation between the oscillatory motion of this cloud after the collision and the correlation between the two electrons of the excited atom is discussed. In ch. 6 it is shown that the broad structures in the non-coincident energy spectra of the Li+-He system are erroneously interpretated as a result of electron emission from the (Li-He)+-quasimolecule. A model is presented which explains, based on the results obtained from the coincidence measurements, these broad structures. In ch. 4 the Post-Collision Interaction process is treated. It is shown that for high-energy collisions, in contrast with general assumptions, PCI is important. In ch. 5 the importance of PCI-processes in photoionization of atoms, followed by Auger decay, are studied. From the formulas derived in ch. 4 simple analytical results are obtained. These are applied to recent experiments and good agreement is achieved. 140 refs.; 55 figs.; 9 tabs

139

Electron-driven excitation and dissociation of molecules  

Science.gov (United States)

Due to the large difference in mass between the electron and the nuclei, when an electron collides with a molecule or molecular ion, there is inefficient transfer of energy from the electron into the motion of the nuclei, leading to little vibrational excitation or dissociation. However, in certain special cases, the electron can temporarily attach to the molecule and change the forces felt between its atoms for a period of time comparable to a vibrational period. This can lead to resonant vibrational excitation and dissociative attachment, for neutral targets, or dissociative recombination in the case of ions. Studies of dissociative recombination and attachment in several polyatomic systems have shown that simple one-dimensional models can fail to capture the correct dissociation dynamics. In our treatment of these processes we first carry out ab initio electron scattering calculations at fixed internuclear geometries to determine the resonant energy surfaces and the corresponding surface of autoionization widths using the Complex Kohn variational method. These resonance positions and widths are then used as input to a dynamics study to determine the cross-section and product distributions for the dissociation or excitation process. We will present results on a number of systems, including HCCH, HCN/HNC and HCCCN as examples of dissociative attachment and N2H^+ and H2O^+ for dissociative recombination.

Orel, Ann

2009-10-01

140

Electron impact excitation of potassium and sodium  

International Nuclear Information System (INIS)

Absolute electron-impact optical excitation functions of 24 transitions of the sharp, principal, diffuse and fundamental series of potassium have been measured in the impact energy range 0 to 400 eV. The determination of the target atom number density was made by measuring the transmissions of potassium resonance radiation from a fluorescence cell upon passage through the collision chamber. Direct excitation cross section of 14 states (5S, 6S, 7S, 8S, 4P, 5P, 6P, 7P, 3D, 5D, 6D, 5F, 6F, and 7F) have been determined from the measured optical excitation cross sections with the aid of radiative transition probabilities taken from the literature. These direct cross section results are compared with theoretical calculations based upon the Born approximation and the multi-state close-couplng approximation. Measurements were also made of the polarization of resolved components of the 4P-4S and 3D-4P doublets, and from these results the direct excitation functions of the separate orbital magnetic substates of the 3P state have been determined. This work has been extended to sodium, yielding measurements of optical excitation functions of 26 transitions of the sharp, principal, diffuse, fundamental, and nP-4S series in the energy range 0 to 150 eV. From these measurements, direct excitation functions of 14 states (4S, 5S, 6S, 7S, 3P, 4P, 5P, 6P, 3D, 4D, 5D, 6D, 6F, and 7F) have been determined. These are compared with theoretical results based on the Born approximation and thets based on the Born approximation and the close-coupling method. Polarization measurements were made for several sharp, principal, and diffuse doublets, and this data is applied to determine the magnetic substate direct substate direct excitation functions for the 3P state

141

Testing time-dependent density functional theory with depopulated molecular orbitals for predicting electronic excitation energies of valence, Rydberg, and charge-transfer states and potential energies near a conical intersection  

International Nuclear Information System (INIS)

Kohn-Sham (KS) time-dependent density functional theory (TDDFT) with most exchange-correlation functionals is well known to systematically underestimate the excitation energies of Rydberg and charge-transfer excited states of atomic and molecular systems. To improve the description of Rydberg states within the KS TDDFT framework, Gaiduk et al. [Phys. Rev. Lett. 108, 253005 (2012)] proposed a scheme that may be called HOMO depopulation. In this study, we tested this scheme on an extensive dataset of valence and Rydberg excitation energies of various atoms, ions, and molecules. It is also tested on a charge-transfer excitation of NH3-F2 and on the potential energy curves of NH3 near a conical intersection. We found that the method can indeed significantly improve the accuracy of predicted Rydberg excitation energies while preserving reasonable accuracy for valence excitation energies. However, it does not appear to improve the description of charge-transfer excitations that are severely underestimated by standard KS TDDFT with conventional exchange-correlation functionals, nor does it perform appreciably better than standard TDDFT for the calculation of potential energy surfaces

142

An excitable electronic circuit as a sensory neuron model  

CERN Document Server

An electronic circuit device, inspired on the FitzHugh-Nagumo model of neuronal excitability, was constructed and shown to operate with characteristics compatible with those of biological sensory neurons. The nonlinear dynamical model of the electronics quantitatively reproduces the experimental observations on the circuit, including the Hopf bifurcation at the onset of tonic spiking. Moreover, we have implemented an analog noise generator as a source to study the variability of the spike trains. When the circuit is in the excitable regime, coherence resonance is observed. At sufficiently low noise intensity the spike trains have Poisson statistics, as in many biological neurons. The transfer function of the stochastic spike trains has a dynamic range of 6 dB, close to experimental values for real olfactory receptor neurons.

Medeiros, Bruno N S; Mindlin, Gabriel B; Copelli, Mauro; Leite, José R Rios

2011-01-01

143

Electronic excitation as a mode of heat dissipation in laser-driven cluster plasmas  

International Nuclear Information System (INIS)

Electrons streaming out of laser plasma are known for non-local heat transport and energy deposition by the ionization wave. At 100 eV electron temperature, since the electronic excitation cross section is comparable to that of ionization for Ar and CO2, a non-local excitation wave akin to the ionization wave is envisaged where energy deposition in excitations forms a excited cluster sheath beyond the laser focus. Here, we show that nano-cluster systems have the right parameters to form such an exciton sheath and experimentally demonstrate this via charge transfer reactions

144

Electronic excitation as a mode of heat dissipation in laser-driven cluster plasmas  

Energy Technology Data Exchange (ETDEWEB)

Electrons streaming out of laser plasma are known for non-local heat transport and energy deposition by the ionization wave. At 100 eV electron temperature, since the electronic excitation cross section is comparable to that of ionization for Ar and CO{sub 2}, a non-local excitation wave akin to the ionization wave is envisaged where energy deposition in excitations forms a excited cluster sheath beyond the laser focus. Here, we show that nano-cluster systems have the right parameters to form such an exciton sheath and experimentally demonstrate this via charge transfer reactions.

Rajeev, R.; Rishad, K. P. M.; Madhu Trivikram, T.; Krishnamurthy, M. [Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-5 (India)

2013-12-15

145

Charge transfer dynamics in molecular solids and adsorbates driven by local and non-local excitations  

Science.gov (United States)

Charge transfer pathways and charge transfer times in molecular films and in adsorbate layers depend both on the details of the electronic structure as well as on the degree of the initial localization of the propagating excited electronic state. For C6F6 molecular adsorbate films on the Cu(111) surface we determined the interplay between excited state localization and charge transfer pathways. In particular we selectively prepared a free-particle-like LUMO band excitation and compared it to a molecularly localized core-excited C1s ? ?* C6F6 LUMO state using time-resolved two-photon photoemission (tr-2PPE) and core-hole-clock (CHC) spectroscopy, respectively. For the molecularly localized core-excited C1s ? ?* C6F6 LUMO state, we separate the intramolecular dynamics from the charge transfer dynamics to the metal substrate by taking the intramolecular dynamics of the free C6F6 molecule into account. Our analysis yields a generally applicable description of charge transfer within molecular adsorbates and to the substrate.

Föhlisch, A.; Vijayalakshmi, S.; Pietzsch, A.; Nagasono, M.; Wurth, W.; Kirchmann, P. S.; Loukakos, P. A.; Bovensiepen, U.; Wolf, M.; Tchaplyguine, M.; Hennies, F.

2012-06-01

146

Intrinsic and dynamical reaction pathways of an excited state proton transfer.  

Science.gov (United States)

The detailed knowledge of excited state proton transfer mechanisms in complex environments is of paramount importance in chemistry. However, the definition of an effective reaction coordinate and the understanding of the driving force of the reaction can be difficult from both the experimental and the theoretical points of view. Here we analyzed by theoretical approaches the mechanism and the driving forces of the excited state proton transfer reaction occurring between the 7-hydroxy-4-(trifluoromethyl)coumarin photoacid and the 1-methylimidazole base molecules in toluene solution. In particular, we compared the intrinsic and the dynamical reaction pathways, obtained by integrating the reaction coordinate, and by performing ab initio simulations of molecular dynamics, respectively. Time-dependent density functional theory and polarizable solvation continuum models were adopted to define the excited state potential energy surface. Results were analyzed by means of the DCT electronic density based index. Our findings suggest that the reaction coordinate is mainly composed of several intra- and intermolecular modes of the reactants. An analysis of both the intrinsic coordinate and the dynamical results shows that the charge transfer induced by electronic excitation of the coumarin molecule is the main proton transfer driving force. With regards to the methodological validation, the combination of ab initio molecular dynamics with time-dependent density functional theory appears to be feasible and reliable to study excited state proton transfer reactions in the condensed phase. PMID:25517266

Raucci, Umberto; Savarese, Marika; Adamo, Carlo; Ciofini, Ilaria; Rega, Nadia

2015-02-12

147

Search for excited electrons through ?? scattering  

Science.gov (United States)

We study the potential of the ?? option of future high-energy linear e+e- colliders to search for excited electrons with spin-1/2. We calculate single production cross sections and give the angular distributions and f-m* contour plots for s=0.5TeV and s=3TeV both using the standard (tree-level) couplings and anomal couplings.

Ozansoy, A.; Billur, A. A.

2012-09-01

148

Probing electronic excitations in molecular conduction  

CERN Document Server

We identify experimental signatures in the current-voltage (I-V) characteristics of weakly contacted molecules directly arising from excitations in their many electron spectrum. We calculate transport properties using a multielectron master equation in the Fock space of an exact diagonalized model many-body Hamiltonian benchmarked for a prototypical molecule (benzene). Using this approach, we quantitatively explain various unique features of molecular conduction that are difficult to capture even qualitatively using standard one-electron self-consistent field (SCF) approaches.

Muralidharan, B; Pati, S K; Datta, S

2005-01-01

149

Excitation of Hydrogen Atoms by Electron Impact  

Science.gov (United States)

We discuss a general theory for electron and hydrogen-like atom collisions. A scaling transformation, under which the time-dependent Schrödinger equation and the plane waves are Galilean invariant, is used. The exact wave functions associated with the initial and final conditions are written in the Fourier space in the sum of the exact initial and final wave functions of the colliding system in addition to the scattering part which is expanded in terms of outgoing-wave Sturmian functions. In this work we investigate the excitation of hydrogen atoms by electron impact in the framework of the Temkin-Poet model.

Tantawi, R. S.; Ovchinnikov, S. Yu.; Macek, J. H.

1998-05-01

150

Excitation energy transfer between closely spaced multichromophoric systems: Effects of band mixing and intraband relaxation  

CERN Document Server

We theoretically analyze the excitation energy transfer between two closely spaced linear molecular J-aggregates, whose excited states are Frenkel excitons. The aggregate with the higher (lower) exciton band edge energy is considered as the donor (acceptor). The celebrated theory of F\\"orster resonance energy transfer (FRET), which relates the transfer rate to the overlap integral of optical spectra, fails in this situation. We point out that in addition to the well-known fact that the point-dipole approximation breaks down (enabling energy transfer between optically forbidden states), also the perturbative treatment of the electronic interactions between donor and acceptor system, which underlies the F\\"orster approach, in general loses its validity due to overlap of the exciton bands. We therefore propose a nonperturbative method, in which donor and acceptor bands are mixed and the energy transfer is described in terms of a phonon-assisted energy relaxation process between the two new (renormalized) bands. ...

Didraga, C; Knöster, J

2006-01-01

151

Electron excitation of Ce3+ in third group metal oxides  

Science.gov (United States)

Among rare-earth ions Ce3+ has the simplest electron configuration - 4f1. Its 4f-5d transitions have an energy of approximately 6.3 eV for a free ion and are quite allowed. Thus, the introduction of Ce3+ into the crystal with wide enough forbidden gap makes it possible to analyze the lattice environment influence on the excited state. During the present research the following results were obtained via spectral-kinetic investigations of the excited states of Ce3+ impurity in various third group metal oxides: (a) luminescence spectrum of Ce3+ has a doublet structure in all examined oxides. This happens due to the energy split of 2F5/2 and 2F7/2 terms of Ce3+ ground state by reason of spin-orbit interaction. (b) In Al2O3 a broad 5-component band corresponding to 4f-5d transitions is observed in the long- wave region of Ce3+ luminescence excitation spectrum. In the short-wave region of excitation spectrum, one can eliminate a non-structured band, which hypothetically consists of the exciton band and the charge transfer band, and as well as an outstanding narrow exciton-band. (c) In YAP bands of 4f-5d transitions are divided into two groups in excitation spectrum: 3-component in the region of 4-5 eV and 2-component in the region of 5-6 eV. (d) In YaG 4f-5d transitions appear as separate bands in excitation spectrum: 2.9, 3.9, 4.6, 5.6, 6.6 eV. Due to the closeness of the fundamental absorption edge in YAG, the band 6.6 eV is observable only at low temperatures. (e) At temperatures under 100 K the energy transfer form exciton to impurity is not detected for all the crystals.

Namozov, B. R.; Vetrov, V. A.; Muradov, S. M.; Zakharchenya, R. I.

2002-07-01

152

Mechanism of Foerster-type hopping of charge transfer and excitation energy transfer along blocked oligothiophenes by Si-atoms  

International Nuclear Information System (INIS)

The ground and excited-state properties of oligothiophenes connected by Si-atoms have been studied theoretically, based on recent experimental reports [M. Fujitsuka, D.W. Cho, J. Ohshita, A. Kunai, T. Majima, J. Phys. Chem. C 111 (2007) 1993]. Herein, we have employed a density-functional theory (DFT) approach toward examining the influence of the number of oligothiophenes on molecular ground-state properties, focusing on the density of state and the density distribution on the subunit of the oligothiophenes. Furthermore, we have investigated several excited-state properties of these oligothiophene. We discuss absorption with transition densities, which reveal the orientations and strengths of transition dipole moments, charge difference densities, which allow for the study of transition dipole moments and charge transfer in the absorption processes, and transition density matrices, which provide information about the electron-hole coherence and excitation delocalization. All of these properties were studied by employing time-dependent density functional theory (TD-DFT). Our theoretical results indicate that there are not only localized excited states, but also inter-branched charge transfer excited states in absorption for block copolymers of the oligothiophenes. In all, the theoretical analyses provide insight into the ground- and excited-state properties of the polymers, notably on the hopping mechanism of charge transfer in blocked oligothiophenes by Si atomsn blocked oligothiophenes by Si atoms

153

Efficient utilization of higher-lying excited states to trigger charge-transfer events.  

Science.gov (United States)

Several new fullerene-heptamethine conjugates, which absorb as far as into the infrared spectrum as 800 nm, have been synthesized and fully characterized by physicochemical means. In terms of optical and electrochemical characteristics, appreciable electronic coupling between both electroactive species is deduced. The latter also reflect the excited-state features. To this end, time-resolved, transient absorption measurements revealed that photoexcitation is followed by a sequence of charge-transfer events which evolve from higher singlet excited states (i.e., S(2)--fast charge transfer) and the lowest singlet excited state of the heptamethine cyanine (i.e., S(1)--slow charge transfer), as the electron donor, to either a covalently linked C(60) or C(70), as the electron acceptor. Finally, charge transfer from photoexcited C(60)/C(70) completes the charge-transfer sequence. The slow internal conversion within the light-harvesting heptamethine cyanine and the strong electronic coupling between the individual constituents are particularly beneficial to this process. PMID:20661968

Bouit, Pierre-Antoine; Spänig, Fabian; Kuzmanich, Gregory; Krokos, Evangelos; Oelsner, Christian; Garcia-Garibay, Miguel A; Delgado, Juan Luis; Martín, Nazario; Guldi, Dirk M

2010-08-16

154

Double, Rydberg and charge transfer excitations from pairing matrix fluctuation and particle-particle random phase approximation  

International Nuclear Information System (INIS)

Double, Rydberg, and charge transfer (CT) excitations have been great challenges for time-dependent density functional theory (TDDFT). Starting from an (N ± 2)-electron single-determinant reference, we investigate excitations for the N-electron system through the pairing matrix fluctuation, which contains information on two-electron addition/removal processes. We adopt the particle-particle random phase approximation (pp-RPA) and the particle-particle Tamm-Dancoff approximation (pp-TDA) to approximate the pairing matrix fluctuation and then determine excitation energies by the differences of two-electron addition/removal energies. This approach captures all types of interesting excitations: single and double excitations are described accurately, Rydberg excitations are in good agreement with experimental data and CT excitations display correct 1/R dependence. Furthermore, the pp-RPA and the pp-TDA have a computational cost similar to TDDFT and consequently are promising for practical calculations

155

Excitation transfer and luminescence in porphyrin-carbon nanotube complexes  

CERN Document Server

Functionalization of carbon nanotubes with hydrosoluble porphyrins (TPPS) is achieved by "$\\pi$-stacking". The porphyrin/nanotube interaction is studied by means of optical absorption, photoluminescence and photoluminescence excitation spectroscopies. The main absorption line of the porphyrins adsorbed on nanotubes exhibits a 120 meV red shift, which we ascribe to a flattening of the molecule in order to optimize $\\pi-\\pi$ interactions. The porphyrin-nanotube complex shows a strong quenching of the TPPS emission while the photoluminescence intensity of the nanotubes is enhanced when the excitation laser is in resonance with the porphyrin absorption band. This reveals an efficient excitation transfer from the TPPS to the carbon nanotube.

Magadur, G; Alain-Rizzo, V; Voisin, C; Roussignol, Ph; Deleporte, E; Delaire, J A

2007-01-01

156

Single-molecule kinetics of interfacial electron transfer  

Energy Technology Data Exchange (ETDEWEB)

Measurements of single-molecule chemical reaction kinetics are demonstrated for interfacial electron transfer from excited cresyl violet molecules to the conduction band of indium tin oxide (ITO) or energetically accessible surface electronic states under ambient conditions by using a far-field fluorescence microscope. In this system, each single molecule exhibits a single-exponential electron transfer kinetics. A wide distribution of site-specific electron transfer rates is observed for many single cresyl violet molecules examined. This work reveals that the physical origin of multiexponential kinetics of electron transfer in this system is the inhomogeneity of molecular interactions on the semiconductor surface of ITO. We illustrate that the single-molecule experiments provide detailed information not obtainable from experiments conducted on large ensembles of molecules. Single-molecule kinetics is particularly useful in understanding multiexponential behavior of chemical reactions in heterogeneous systems. 42 refs., 4 figs.

Lu, H.P.; Xie, X.S. [Pacific Northwest National Lab., Richland, WA (United States)

1997-04-10

157

Electron-impact excitation of Ne+  

International Nuclear Information System (INIS)

We present the results of a 61-term, 138-level intermediate-coupling frame-transformation R-matrix close-coupling calculation of the electron-impact excitation of fluorine-like Ne+. All levels of the 2s22p5, 2s2p6, 2s22p43l and 2s22p44l configurations that lie below the ionization limit are included in the close-coupling expansion. With the exception of several R-matrix calculations of excitation between the fine structure levels of 2s22p5 2P, this represents the first close-coupling calculation for this ion. Here we describe this calculation and present radiative rates and effective collision strengths for a selected number of the 9453 transitions resulting from this work. The full set of data is available at the Oak Ridge National Laboratory Controlled Fusion Atomic Data Center Web site. (author)

158

75 FR 66644 - Electronic Fund Transfers  

Science.gov (United States)

...exceptions for any class of electronic fund transfers that in the...responsibilities of participants in electronic fund transfer systems...final rule, the Board found good cause under 5 U.S.C...205 Consumer protection, Electronic fund transfers, Federal...

2010-10-29

159

Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria  

CERN Document Server

Chlorosomes are the largest and most efficient natural light-harvesting antenna systems. They contain thousands of pigment molecules - bacteriochlorophylls (BChls)- that are organized into supramolecular aggregates and form a very efficient network for excitonic energy migration. Here, we present a theoretical study of excitation energy transfer (EET) in the chlorosome based on experimental evidence of the molecular assembly. Our model for the exciton dynamics throughout the antenna combines a stochastic time propagation of the excitonic wave function with molecular dynamics simulations of supramolecular structure, and electronic structure calculations of the excited states. The simulation results reveal a detailed picture of the EET in the chlorosome. Coherent energy transfer is significant only for the first 50 fs after the initial excitation, and the wavelike motion of the exciton is completely damped at 100 fs. Characteristic time constants of incoherent energy transfer, subsequently, vary from 1 ps to se...

Fujita, Takatoshi; Saikin, Semion K; Brookes, Jennifer C; Aspuru-Guzik, Alan

2013-01-01

160

A benchmark study of electronic excitation energies, transition moments, and excited-state energy gradients on the nicotine molecule.  

Science.gov (United States)

In this work, we report a comparative study of computed excitation energies, oscillator strengths, and excited-state energy gradients of (S)-nicotine, chosen as a test case, using multireference methods, coupled cluster singles and doubles, and methods based on time-dependent density functional theory. This system was chosen because its apparent simplicity hides a complex electronic structure, as several different types of valence excitations are possible, including n-?(*), ?-?(*), and charge-transfer states, and in order to simulate its spectrum it is necessary to describe all of them consistently well by the chosen method. PMID:25494739

Egidi, Franco; Segado, Mireia; Koch, Henrik; Cappelli, Chiara; Barone, Vincenzo

2014-12-14

161

A benchmark study of electronic excitation energies, transition moments, and excited-state energy gradients on the nicotine molecule  

Science.gov (United States)

In this work, we report a comparative study of computed excitation energies, oscillator strengths, and excited-state energy gradients of (S)-nicotine, chosen as a test case, using multireference methods, coupled cluster singles and doubles, and methods based on time-dependent density functional theory. This system was chosen because its apparent simplicity hides a complex electronic structure, as several different types of valence excitations are possible, including n-?*, ?-?*, and charge-transfer states, and in order to simulate its spectrum it is necessary to describe all of them consistently well by the chosen method.

Egidi, Franco; Segado, Mireia; Koch, Henrik; Cappelli, Chiara; Barone, Vincenzo

2014-12-01

162

Double Resonance Studies of Electronically Excited Acetylene.  

Science.gov (United States)

The double resonance techniques of Ultraviolet ^{-}Ultraviolet Double Resonance (UVUVDR), Ultraviolet-Optical Double Resonance (UVODR), and Stimulated Emission Pumping (SEP) have been used to investigate electronically excited states of acetylene. Electronically excited states of acetylene have stable structures in the linear (D_{infty h}), cis-bent (C_sp{2v}{b}) , trans-bent (C_sp{2h} {c}), vinylidene (C_sp {2v}{a}), and nonplanar (C _sp{2}{b} and C _sp{2}{c}) configurations, all of which might appear in the spectrum. The Complete Nuclear Permutation Inversion-Molecular Symmetry (CNPI -MS) group theoretical treatment was utilized (Chapter 2) to provide a consistent and point group independent framework with which to classify the overall rovibronic wavefunctions connected with these different structures. Knowing the unique patterns of rotational energy levels presented by the different isomeric structures of acetylene is instrumental in determining the electronic symmetry and molecular structure of the states appearing in the spectrum. Although the excited states in the 60,000-78,000 cm^{-1} energy region are strongly predissociated, the UVUVDR and UVODR double resonance techniques permit the recording the rotationally resolved spectra. The rotationally resolved spectra allow the complete characterization of these states, which in a one-photon absorption spectrum can only be classified as diffuse. The most important result of these double resonance studies is the structural and electronic symmetry characterization of two predissociated electronic states (Chapters 3 and 4), most notably the nonplanar ~ E -state. Confirmation of this nonplanar gauche structure relies heavily on the observation of an unprecedented K_{a} dependent pattern of rotational selection rules. The rotational energy level pattern exhibited by the ~ E -state is consistent with that expected (Chapter 2) for a nonplanar near cis-bent (C_sp{2} {b}) state of ^1 A electronic symmetry. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.).

Lundberg, James Kenneth

1992-01-01

163

Photoinduced electron transfer processes in homogeneous and microheterogeneous solutions  

Energy Technology Data Exchange (ETDEWEB)

The focus of the work described in this report is on single electron transfer reactions of excited states which culminate in the formation of stable or metastable even electron species. For the most part the studies have involved even electron organic substrates which are thus converted photochemically to odd electron species and then at some stage reconvert to even electron products. These reactions generally fall into two rather different categories. In one set of studies we have examined reactions in which the metastable reagents generated by single electron transfer quenching of an excited state undergo novel fragmentation reactions, chiefly involving C-C bond cleavage. These reactions often culminate in novel and potentially useful chemical reactions and frequently have the potential for leading to new chemical products otherwise unaffordable by conventional reaction paths. In a rather different investigation we have also studied reactions in which single electron transfer quenching of an excited state is followed by subsequent reactions which lead reversibly to metastable two electron products which, often stable in themselves, can nonetheless be reacted with each other or with other reagents to regenerate the starting materials with release of energy. 66 refs., 9 figs., 1 tab.

Whitten, D.G.

1991-10-01

164

Excitation energy transfer processes in condensed matter theory and applications  

CERN Document Server

Applying a unified quantum approach, contributors offer fresh insights into the theoretical developments in the excitation energy transfer processes in condensed matter This comprehensive volume examines Frenkel and Wannier excitonic processes; rates of excitonic processes; theory of laser sputter and polymer ablation; and polarons, excitonic polarons and self-trapping

Singh, Jai

1994-01-01

165

Excitation energy transfer: Study with non-Markovian dynamics  

International Nuclear Information System (INIS)

In this paper, we investigate the non-Markovian dynamics of a model to mimic the excitation energy transfer (EET) between chromophores in photosynthesis systems. The numerical path integral method is used. This method includes the non-Markovian effects of the environmental affects, and it does not need the perturbation approximation in solving the dynamics of systems of interest. It implies that the coherence helps the EET between chromophores through lasting the transfer time rather than enhancing the transfer rate of the EET. In particular, the non-Markovian environment greatly increases the efficiency of the EET in the photosynthesis systems.

166

Electron impact excitation of copper atoms  

International Nuclear Information System (INIS)

The optical excitation function method has been used in a crossed atom and electron beam arrangement to measure the electron impact cross section of the copper 42P ? 42S resonance lines (324.8, 327.4 nm) from threshold (3.8 eV) to 8 eV. Relative experimental cross section data are normalized at an energy of 1000 eV with respect to first Born theory that includes the 42S ? 42P resonance transition with an oscillator strength of 0.652 and cascading from the (3d10nd)2D states with n = 4, hor-ellipsis 10. The measured Cu 42S4 ? 42P cross section is compared with recent theoretical calculations in close-coupling approximation. Very good agreement is found with the ten-state close-coupling theory of Scheibner

167

Calculation of nuclear excitation in an electron transition  

International Nuclear Information System (INIS)

We have made a theoretical investigation of nuclear excitation during an electron transition (NEET). Our approach allows us to express the NEET probabilities in terms of the excited nuclear level width, the energy difference between the nuclear and electron transition, the Coulomb interaction between the initial electron states, and the electron level width. A comparison is made with the available experimental results. (orig.)

168

Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation.  

Science.gov (United States)

The electronic spectroscopy and the electronic excited state properties of cold protonated phenylalanine and protonated tyrosine have been revisited on a large spectral domain and interpreted by comparison with ab initio calculations. The protonated species are stored in a cryogenically cooled Paul trap, maintained at ?10 K, and the parent and all the photofragment ions are mass-analyzed in a time-of-flight mass spectrometer, which allows detecting the ionic species with an improved mass resolution compared to what is routinely achieved with a quadrupole mass spectrometer. These new results emphasize the competition around the band origin between two proton transfer reactions from the ammonium group toward either the aromatic chromophore or the carboxylic acid group. These reactions are initiated by the coupling of the locally excited ??* state with higher charge transfer states, the positions and coupling of which depend on the conformation of the protonated molecules. Each of these reaction processes gives rise to specific fragmentation channels that supports the conformer selectivity observed in the photofragmentation spectra of protonated tyrosine and phenylalanine. PMID:25248102

Féraud, Géraldine; Broquier, Michel; Dedonder, Claude; Jouvet, Christophe; Grégoire, Gilles; Soorkia, Satchin

2014-09-23

169

Fundamental studies of interfacial excited-state charge transfer in molecularly tethered semiconductor nanoassemblies  

Science.gov (United States)

The research presented in this dissertation focuses on elucidating the parameters affecting dynamics and yield of electron transfer reactions in semiconducting nanoparticle assemblies through the use of time-resolved spectroscopy. In particular, the dissertation focuses on photoinduced electron injection in assemblies of CdSe, CdS, or PbS quantum dots covalently bound to either metal oxide films or each other through the use of bifunctional molecular linkers. Chapter 2 elucidates the influence of electronic coupling on excited-state electron transfer from CdS quantum dots to TiO2 nanoparticles via molecular linkers with phenylene bridges. We establish that the efficiency of electron injection from CdS quantum dots to TiO2 nanoparticle varies dramatically with electronic coupling, which can be controlled by tuning the properties of molecular linkers. Chapter 3 presents the role of excitation energy on interfacial electron transfer in tethered assemblies of CdSe quantum dots and TiO2 nanoparticles. Through this work, we determined that injection efficiency from band-edge states is independent of excitation energy. However, the efficiency of injection from trap-states decreases at lower-energy excitation. We attribute the decrease to a lower energy distribution of emissive trap-states from which injection is less efficient. Chapter 4 presents the observation of multiphasic electron injection dynamics from photoexcited PbS quantum dots to TiO2 nanoparticles. In this collaborative study with Dr. Masumoto from the University of Tsukuba we observed electron injection on multiple timescales. We determined that electron injection occurred in this system through two different mechanisms. The first involved injection from thermalized PbS excited states and the second through injection of hot electrons through Auger recombination of biexcitons that creates high lying excitonic states. Chapter 5 investigates charge transfer in covalently bound quantum dot assemblies. We utilize established carbodiimide chemistry to tether quantum dots to each other through covalent bonds between capping groups. As a result of our controlled-covalent quantum dot attachment, we observe evidence of interfacial charge transfer from photoexcited CdS quantum dots to covalently-bound CdSe quantum dots.

Youker, Diane Greer

170

Collisionally excited few-electron systems: theoretical introduction and survey  

International Nuclear Information System (INIS)

We consider excitation, ionization, and charge transfer in collisions of protons (and antiprotons) with the single-electron targets H, He+, and Li2+. These collisions are first compared to other types of ion-atom collisions. A brief review of our own theoretical method is given; in particular we describe how we allow for both large charge transfer and ionization probabilities while retaining the computational efficiency that allows us to consider a variety of collision partners and collision energies. We comment on the comparison of our results to other theoretical work and to experiment. The qualitative features of the various inelastic cross sections are discussed, in particular how they scale with collision energy, target nuclear charge, and the sign of the projectile charge. 15 references, 6 figures

171

Selective-excitation study of collisional excitation transfer for helium 3sup(1,3)P and D states  

International Nuclear Information System (INIS)

By observing the time-resolved fluorescence following the laser excitation of a helium level in a discharge plasma, the authors have determined the excitation transfer rate coefficients among the n = 3 levels due to atomic and electronic collisions. The result in the form of the thermally averaged cross section is: for 31P ? 31D = (1.9 +- 0.4) x 10-15 cm2 and = (2.4 +- 1.2) x 10-13 cm2; for 33D ? 33P = (5.3 +- 1.2) x 10-17 cm2 and = (0.9 +- 0.4) x 10-13 cm2. The cross sections for atomic collisions are in reasonably good agreement with other experimental and theoretical data, but the cross sections for electronic collisions are larger than the earlier experimental data by two orders and are in extremely good agreement with the recent calculations. (author)

172

Resonant hot charge-transfer excitations in fullerene-porphyrin complexes: Many-body Bethe-Salpeter study  

Science.gov (United States)

We study the neutral singlet excitations of the zinc-tetraphenylporphyrin and C70-fullerene donor-acceptor complex within the many-body Green's-function GW and Bethe-Salpeter approaches. The lowest transition is a charge-transfer excitation between the donor and the acceptor with an energy in excellent agreement with recent constrained density functional theory calculations. Beyond the lowest charge-transfer state, which can be determined with simple electrostatic models that we validate, the Bethe-Salpeter approach provides the full excitation spectrum. We evidence the existence of hot electron-hole states which are resonant in energy with the lowest donor intramolecular excitation and show a hybrid intramolecular and charge-transfer character, favoring the transition towards charge separation. Such findings, and the ability to describe accurately both low-lying and excited charge-transfer states, are important steps in the process of discriminating “cold” versus “hot” exciton dissociation processes.

Duchemin, Ivan; Blase, Xavier

2013-06-01

173

Light induced electron transfer reactions of metal complexes  

Energy Technology Data Exchange (ETDEWEB)

Properties of the excited states of tris(2,2'-bipyridine) and tris(1,10-phenanthroline) complexes of chromium(III), iron(II), ruthenium(II), osmium(II), rhodium(III), and iridium(III) are described. The electron transfer reactions of the ground and excited states are discussed and interpreted in terms of the driving force for the reaction and the distortions of the excited states relative to the corresponding ground states. General considerations relevant to the conversion of light into chemical energy are presented and progress in the use of polypyridine complexes to effect the light decomposition of water into hydrogen and oxygen is reviewed.

Sutin, N; Creutz, C

1980-01-01

174

Light induced electron transfer reactions of metal complexes  

International Nuclear Information System (INIS)

Properties of the excited states of tris(2,2'-bipyridine) and tris(1,10-phenanthroline) complexes of chromium(III), iron(II), ruthenium(II), osmium(II), rhodium(III), and iridium(III) are described. The electron transfer reactions of the ground and excited states are discussed and interpreted in terms of the driving force for the reaction and the distortions of the excited states relative to the corresponding ground states. General considerations relevant to the conversion of light into chemical energy are presented and progress in the use of polypyridine complexes to effect the light decomposition of water into hydrogen and oxygen is reviewed

175

An Accurate and Linear Scaling Method to Calculate Charge-Transfer Excitation Energies and Diabatic Couplings  

CERN Document Server

Quantum--Mechanical methods that are both computationally fast and accurate are not yet available for electronic excitations having charge transfer character. In this work, we present a significant step forward towards this goal for those charge transfer excitations that take place between non-covalently bound molecules. In particular, we present a method that scales linearly with the number of non-covalently bound molecules in the system and is based on a two-pronged approach: The molecular electronic structure of broken-symmetry charge-localized states is obtained with the Frozen Density Embedding formulation of subsystem Density-Functional Theory; subsequently, in a post-SCF calculation, the full-electron Hamiltonian and overlap matrix elements among the charge-localized states are evaluated with an algorithm which takes full advantage of the subsystem DFT density partitioning technique. The method is benchmarked against Coupled-Cluster calculations and achieves chemical accuracy for the systems considered...

Pavanello, Michele; Visscher, Lucas; Neugebauer, Johannes

2012-01-01

176

The effect of atoms excited by electron beam on metal evaporation  

CERN Document Server

In atomic vapor laser isotope separation (AVLIS), the metal is heated to melt by electron beams. The vapor atoms may be excited by electrons when flying through the electron beam. The excited atoms may be deexcited by inelastic collision during expansion. The electronic energy transfers translational energy. In order to analyse the effect of reaction between atoms and electron beams on vapor physical parameters, such as density, velocity and temperature, direct-simulation Monte Carlo method (DSMC) is used to simulate the 2-D gadolinium evaporation from long and narrow crucible. The simulation results show that the velocity and temperature of vapor increase, and the density decreases

Xie Guo Feng; Ying Chun Tong

2002-01-01

177

Laser pulse control of bridge mediated heterogeneous electron transfer  

International Nuclear Information System (INIS)

Ultrafast heterogeneous electron transfer from surface attached dye molecules into semiconductor band states is analyzed. The focus is on systems where the dye is separated from the surface by different bridge anchor groups. To simulate the full quantum dynamics of the transfer process a model of reduced dimensionality is used. It comprises the electronic levels of the dye, the bridge anchor group electronic levels and the continuum of semiconductor band states, all defined versus a single intramolecular vibrational coordinate. The effect of the bridge states is demonstrated, firstly, in studying the injection dynamics following an impulsive excitation of the dye. Then, by discussing different control tasks it is demonstrate in which way the charge injection process can be influenced by tailored laser pulses. To highlight the importance of electron wave function interference emphasis is put on asymmetric two-bridge molecule systems which are also characterized by different and complex valued electronic transfer matrix elements.

178

Laser pulse control of bridge mediated heterogeneous electron transfer  

Science.gov (United States)

Ultrafast heterogeneous electron transfer from surface attached dye molecules into semiconductor band states is analyzed. The focus is on systems where the dye is separated from the surface by different bridge anchor groups. To simulate the full quantum dynamics of the transfer process a model of reduced dimensionality is used. It comprises the electronic levels of the dye, the bridge anchor group electronic levels and the continuum of semiconductor band states, all defined versus a single intramolecular vibrational coordinate. The effect of the bridge states is demonstrated, firstly, in studying the injection dynamics following an impulsive excitation of the dye. Then, by discussing different control tasks it is demonstrate in which way the charge injection process can be influenced by tailored laser pulses. To highlight the importance of electron wave function interference emphasis is put on asymmetric two-bridge molecule systems which are also characterized by different and complex valued electronic transfer matrix elements.

Wang, Luxia; May, Volkhard

2009-06-01

179

Laser-assisted nuclear ? excitation by the inverse electronic-bridge process  

International Nuclear Information System (INIS)

A method to generate low-energy and high-multipolarity nuclear transitions through a laser-assisted, resonant, inverse electronic-bridge process is presented. The cross section of the suggested nuclear-excitation process is calculated in a simple model. The yield is compared with the yields of ordinary ?-ray absorption and Coulomb excitation. The excitation of the 235mU isomeric state of energy 73.5 eV by an E3 transition is traced numerically. The excitation of an electron from the O4 (5d3/2) electronic shell of binding energy 105 eV can be followed by a P3 (6p3/2)?O4 transition, which can be tuned by the laser to resonance with the nuclear transition. In case of resonance, the electronic excitation energy is transferred to the nucleus with a high efficiency

180

Electronic Excitation of Furan by Low Energy Electrons  

Science.gov (United States)

We present absolute differential cross section (DCS) measurements and calculations of electron impact excitation of the lowest lying triplet ^3B2 and ^3A1 electronic states of furan. The incident electron energy range of the present study was 5-15eV. The experimental data were normalized to the elastic DCS data of [1]. The cross sections were determined by unfolding electron energy loss spectra, using an open source data analysis package and the spectroscopic assignments of [2]. The calculations employ a Multichannel Schwinger method with a 9-state closed coupling CI configuration including polarized pseudo-potentials. The preliminary theoretical results show reasonable agreement with experiment below 10eV, but differ at higher energies. [4pt] [1] M. A. Khakoo et al., Phys. Rev. A, 81, 062716 (2010) [0pt] [2] A. Guiliani and M.-J. Hubin-Franskin, Int. J. Mass Spec., 205, 163 (2001)

Hargreaves, Leigh R.; Khakoo, Murtadha A.; Lopes, Maria Cristina A.; da Costa, Romarly; Bettega, Marcio H. F.; Lima, Marco A. P.

2011-11-01

181

Electron Impact Excitation of S III  

Science.gov (United States)

We present collision strengths and effective collision strengths for the electron impact excitation of S,. The paralell RMATRX II suite of codes have been used, which perform the calculation in intermediate coupling, and we have incorporated 29 LS states in our calculation, which gives rise to 53 fine structure levels and a total of 1378 transitions. Collision strengths have been generated over an electron energy range of 0-12,yd, and from these effective collision strength data are determined for electron temperatures in the range log10T(K)=3.0-6.0. Results are given for transitions between the the fine structure levels within the ground state configuration of 3s^23p^2. Comparisons are made with the previous R-matrix calculations of Galav'is, Mendoza & Zeippen [1] (carried out as part of the IRON Project) and that of Tayal & Gupta [2]. Our current work helps to resolve a large discrepancy which existed between the these two earlier calculations for some of the data within the ground state transitions.[4pt] 1] Galav'is ME, Mendoza C, & Zeippen CJ 1995, Astron. Astrophys. Suppl. Ser. 111 347;[0pt] [2] Tayal SS & Gupta GP 1999 ApJ 526 544.

Hudson, Claire; Ramsbottom, Cathy

2011-06-01

182

Detection of ground state atoms by electron excitation to a metastable level  

International Nuclear Information System (INIS)

A method of detecting ground state atoms in a molecular beam, following electron excitation to a metastable state, is described. Using beams at right angles the momentum transfer and the spread in the momentum transfer are measured as functions of the electron energy. An experimental design for such an atom detector, for use with noble gases, is derived and the solid angle acceptance of the detector, which directly determines the amount of background suppression, is determined. (U.K.)

183

Patterning of insulating surfaces by electronic excitation  

International Nuclear Information System (INIS)

The interaction of swift heavy ions with any material is governed by a strong electronic excitation. In insulators this often leads to permanent modifications. We have irradiated various insulating materials (SrTiO3,TiO2,Al2O3,CaF2,SrF2,BaF2,andLiF) with swift heavy ions and have studied the irradiated surfaces by means of scanning force microscopy. For all materials used in this study we find one nanosized hillock per incident ion if the beam is oriented perpendicular with respect to the surface. Under oblique angles we find elongated chains of discontinuous nanodots on all crystalline materials investigated here.

184

Spectral analysis of electron transfer kinetics II  

OpenAIRE

Electron transfer processes in Debye solvents are studied using a spectral analysis method recently proposed. Spectral structure of a nonadiabatic two-state diffusion equation is investigated to reveal various kinetic regimes characterized by a broad range of physical parameters; electronic coupling, energy bias, reorganization energy, and solvent relaxation rate. Within this unified framework, several kinetic behaviors of the electron transfer kinetics, including adiabatic ...

Jung, Younjoon; Cao, Jianshu

2002-01-01

185

Electron excitations due to plastic deformation of ionic crystals  

International Nuclear Information System (INIS)

Electron emission and luminescence accompanying plastic deformation of Li F and Na F single crystals are investigated. It is shown that intersection of dislocations can result in electron excitations. Strain-induced electron emission and luminescence are stated to be conditioned by these excitations

186

Electron Transfer Dissociation of Oligonucleotide Cations  

OpenAIRE

Electron transfer dissociation (ETD) of multi-protonated 6 - 20-mer oligonucleotides and 12- and 14-mer duplexes is compared to collision activated dissociation (CAD). ETD causes efficient charge reduction of the multi-protonated oligonucleotides in addition to limited backbone cleavages to yield sequence ions of low abundance. Subsequent CAD of the charge-reduced oligonucleotides formed upon electron transfer, in a net process termed electron transfer collision activated dissociation (ETcaD)...

Smith, Suncerae I.; Brodbelt, Jennifer S.

2009-01-01

187

Transfer excitation reactions in fast proton-helium collisions  

OpenAIRE

Continuing previous work, we have measured the projectile scattering-angle dependency for transfer excitation of fast protons (300-1200 keV/u) colliding with helium (p+He $\\rightarrow$ H + He$^{+ *}$). Our high-resolution fully differential data are accompanied by calculations, performed in the plane wave first Born approximation and the eikonal wave Born approximation. Experimentally we find a deep minimum in the differential cross section around 0.5 $mrad$. The comparison ...

Scho?ffler, M. S.; Kim, H. -k; Chuluunbaatar, O.; Houamer, S.; Galstyan, A. G.; Titze, J. N.; Jahnke, T.; Schmidt, L. Ph H.; Schmidt-b Ocking, H.; D Orner, R.; Popov, Yu V.; Bulychev, A. A.

2013-01-01

188

Exploring the metric of excited state proton transfer reactions.  

Science.gov (United States)

The excited state proton transfer (ESPT) reaction taking place between 7-hydroxy-4-(trifluorometyl)coumarin and 1-methylimidazole, recently experimentally characterized, has been here considered as a case study to illustrate the possibility of using theoretical approaches rooted in density functional theory (DFT) and time-dependent DFT (TD-DFT) for the description of complex reactions occurring at the excited state. In particular, beside identifying all stable species occurring at the ground and excited state during the ESPT reaction, a quantitative characterization of their photophysical properties, such as absorption and emission, is obtained by properly including solvent effects. More interestingly, a computational protocol enabling one to locate possible reaction pathways for the ESPT is here proposed. This protocol is based on the use of density based indices purposely developed to characterize the properties of vertical and relaxed excited states which allow one to discriminate the most favorable reaction paths on potential energy surfaces that are in the case of ESPT intrinsically very flat and difficult to characterize based on sole energy criteria, thus opening a new scenario for the description of photoinduced proton transfer reactions. PMID:24251592

Savarese, Marika; Netti, Paolo A; Adamo, Carlo; Rega, Nadia; Ciofini, Ilaria

2013-12-19

189

Distance dependence of excitation energy transfer between spacer-separated conjugated polymer films  

Science.gov (United States)

We report a systematic study of the scaling with distance of electronic energy transfer between thin films of conjugated polymers separated by a silica spacer. The energy-transfer kinetics were obtained directly from time-resolved photoluminescence measurements and show a 1/z3 distance dependence of the transfer rate between the excited donor and the acceptor film for z?8nm . This is consistent with Förster theory; but at shorter separations the energy transfer is slower than predicted and can be explained by the breakdown of the point-dipole approximation at ztilde 5nm . The results are relevant for organic photovoltaics and light-emitting devices, where energy transfer can provide a means of increasing performance.

Shaw, Paul E.; Ruseckas, Arvydas; Samuel, Ifor D. W.

2008-12-01

190

Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria.  

Science.gov (United States)

We present a theoretical study of excitation dynamics in the chlorosome antenna complex of green photosynthetic bacteria based on a recently proposed model for the molecular assembly. Our model for the excitation energy transfer (EET) throughout the antenna combines a stochastic time propagation of the excitonic wave function with molecular dynamics simulations of the supramolecular structure and electronic structure calculations of the excited states. We characterized the optical properties of the chlorosome with absorption, circular dichroism and fluorescence polarization anisotropy decay spectra. The simulation results for the excitation dynamics reveal a detailed picture of the EET in the chlorosome. Coherent energy transfer is significant only for the first 50 fs after the initial excitation, and the wavelike motion of the exciton is completely damped at 100 fs. Characteristic time constants of incoherent energy transfer, subsequently, vary from 1 ps to several tens of ps. We assign the time scales of the EET to specific physical processes by comparing our results with the data obtained from time-resolved spectroscopy experiments. PMID:24504540

Fujita, Takatoshi; Huh, Joonsuk; Saikin, Semion K; Brookes, Jennifer C; Aspuru-Guzik, Alán

2014-06-01

191

Electronic excited states of protonated aromatic molecules: Protonated Fluorene  

Energy Technology Data Exchange (ETDEWEB)

Highlights: Black-Right-Pointing-Pointer We report the vibrationally resolved electronic spectrum of protonated fluorene. Black-Right-Pointing-Pointer The ground and excited states of the x possible isomers have been calculated ab initio. Black-Right-Pointing-Pointer Among these isomers, only two may contribute to the two band systems observed. Black-Right-Pointing-Pointer Franck Condon simulations are used to assign the active vibrations. - Abstract: The photo-fragmentation spectrum of protonated fluorene has been recorded in the visible spectral region, evidencing an absorption that appears largely red shifted in comparison to that of the neutral molecule fluorene. The spectrum shows two different vibrational progressions, separated by 0.19 eV. As in the case of protonated linear polycyclic aromatic hydrocarbons (PAHs), comparison of the measured spectra with ab initio calculations allows to associate the observed absorption shift with the charge transfer character of the excited state. The spectra can be properly simulated by geometry optimization of the ground and excited states, followed by Franck Condon analysis. The two vibrational bands progressions observed are assigned, with relatively good confidence, to the existence of two different conformers.

Alata, Ivan [CLUPS (Centre Laser de l' Universite Paris Sud / LUMAT FR 2764) Bat. 106, Univ Paris-Sud 11, 91405 Orsay Cedex (France) and Atomic Energy Commission of Syria, Damascus, P.O. Box 6091 (Syrian Arab Republic); Broquier, Michel; Dedonder, Claude [CLUPS (Centre Laser de l' Universite Paris Sud / LUMAT FR 2764) Bat. 106, Univ Paris-Sud 11, 91405 Orsay Cedex (France); Institut des Sciences Moleculaires d' Orsay (ISMO, UMR8624 CNRS) Bat. 210, Univ Paris-Sud 11, 91405 Orsay Cedex (France); Jouvet, Christophe, E-mail: christophe.jouvet@u-psud.fr [CLUPS (Centre Laser de l' Universite Paris Sud / LUMAT FR 2764) Bat. 106, Univ Paris-Sud 11, 91405 Orsay Cedex (France); Institut des Sciences Moleculaires d' Orsay (ISMO, UMR8624 CNRS) Bat. 210, Univ Paris-Sud 11, 91405 Orsay Cedex (France); Marceca, Ernesto [INQUIMAE-FCEN, UBA, Ciudad Universitaria, 3er piso, Pab. II, 1428 Buenos Aires (Argentina)

2012-01-17

192

Observation of slow charge redistribution preceding excited-state proton transfer  

International Nuclear Information System (INIS)

The photoacid 8-hydroxy-N,N,N',N',N',N'-hexamethylpyrene-1,3,6-trisulfonamide (HPTA) and related compounds are used to investigate the steps involved in excited-state deprotonation in polar solvents using pump-probe spectroscopy and time correlated single photon counting fluorescence spectroscopy. The dynamics show a clear two-step process leading to excited-state proton transfer. The first step after electronic excitation is charge redistribution occurring on a tens of picoseconds time scale followed by proton transfer on a nanosecond time scale. The three states observed in the experiments (initial excited state, charge redistributed state, and proton transfer state) are recognized by distinct features in the time dependence of the pump-probe spectrum and fluorescence spectra. In the charge redistributed state, charge density has transferred from the hydroxyl oxygen to the pyrene ring, but the OH sigma bond is still intact. The experiments indicate that the charge redistribution step is controlled by a specific hydrogen bond donation from HPTA to the accepting base molecule. The second step is the full deprotonation of the photoacid. The full deprotonation is clearly marked by the growth of stimulated emission spectral band in the pump-probe spectrum that is identical to the fluorescence spectrum of the anion

193

Excited State Intramolecular Proton Transfer and Photophysics of a New Fluorenyl Two-Photon Fluorescent Probe  

OpenAIRE

The steady-state photophysical, NMR, and two-photon absorption (2PA) properties of a new fluorene derivative (1) containing the 2-(2?-hydroxyphenyl) benzothiazole (HBT) terminal construct was investigated for use as a fluorescence probe in bioimaging. A comprehensive analysis of the linear spectral properties revealed inter- and intramolecular hydrogen bonding and excited state intramolecular proton transfer (ESIPT) processes in the HBT substituent. A specific electronic model with a double...

Morales, Alma R.; Schafer-hales, Katherine J.; Yanez, Ciceron O.; Bondar, Mykhailo V.; Przhonska, Olga V.; Marcus, Adam I.; Belfield, Kevin D.

2009-01-01

194

The mechanism of electronic excitation in the bacterial bioluminescent reaction  

International Nuclear Information System (INIS)

The current state of the problem of formation of the electron-excited product in the chemiluminescent reaction that underlies the bacterial luminescence is analysed. Various schemes of chemical transformations capable of producing a bacterial bioluminescence emitter are presented. The problem of excitation of secondary emitters is considered; two possible mechanisms of their excitation are analysed.

195

Multiphoton excitation of doubly excited states of two-electron atoms  

International Nuclear Information System (INIS)

One of the outstanding problems in atomic physics is to better understand the dynamics of highly excited two-electron systems. On the experimental side, single photon excitation studies have been limited to relatively low-lying states in He, while electron scattering experiments do not adequately resolve individual states. This paucity of experimental data is due to three general characteristics of these highly excited double electron states: (1) they have high energies usually greater than 1 Ryd; (2) they have little overlap with the ground state; (3) single photon excitation will not efficiently excite these states. It is the purpose of this paper to discuss the multiphoton excitation of these sperial states in two-electron systems. 12 references, 4 figures

196

Convoy electron emission following ionization of highly-charged ions excited by resonant coherent excitation  

International Nuclear Information System (INIS)

Projectile ionization of highly-charged Ar and Fe ions in the excited states passing relativistically fast through a thin crystalline foil was experimentally studied. We selectively controlled the population of the excited states of the projectiles, and their alignment by choosing a specific m-state through three-dimensional resonant coherent excitation technique by periodical electric fields in a crystalline. We measured energy-differential spectra of electron emission released from projectiles at zero degree. Under the resonance condition, we found an evident enhancement of the convoy electron yield, which reflects the electron momentum distribution of the initial bound state of the excited ions.

197

Electronic excitations in some transition metals and their oxides  

International Nuclear Information System (INIS)

The characteristic energy losses suffered by a 20 keV electron beam through thin films of some transition metals (Sc, Y, Zr, Hf) and of their oxides are measured using a magnetic spectrograph up to 50 eV. The energy loss function -Im 1/epsilon and the complex dielectric constant are calculated. It is shown that the electron excitation spectra of metals contain two main contributions in this energy range: one due to collective excitations of the conduction electrons, the other to the excitation of inner p-electrons. In the case of oxides the spectra display three strong structures: there are probably two collective excitations of valence electrons followed by the excitation of inner p-electrons of the metal. (author)

198

Photoinitiated electron transfer in multi-chromophoric species: Synthetic tetrads and pentads. Technical progress report, 1987--1990  

Energy Technology Data Exchange (ETDEWEB)

This research project involves the design, synthesis and study of the molecules which mimic many of the important aspects of photosynthetic electron and energy transfer. Specifically, the molecules are designed to mimic the following aspects of natural photosynthetic multistep electron transfer: electron donation from a tetrapyrrole excited singlet state, electron transfer between tetrapyrroles, electron transfer from tetrapyrroles to quinones, and electron transfer between quinones with different redox properties. In addition, they model carotenoid antenna function in photosynthesis (singlet-singlet energy transfer from carotenoid polyenes to chlorophyll) and carotenoid photoprotection from singlet oxygen damage (triplet-triplet energy transfer from chlorophyll to carotenoids).

1990-02-14

199

Excited-state intermolecular proton transfer of firefly luciferin V. Direct proton transfer to fluoride and other mild bases.  

Science.gov (United States)

We studied the direct proton transfer (PT) from electronically excited D-luciferin to several mild bases. The fluorescence up-conversion technique is used to measure the rise and decay of the fluorescence signals of the protonated and deprotonated species of D-luciferin. From a base concentration of 0.25 M or higher the proton transfer rates to the fluoride, dihdyrogen phosphate or acetate bases are fast and comparable. The fluorescence signals are nonexponential and complex. We suggest that the fastest decay component arises from a direct proton transfer process from the hydroxyl group of D-luciferin to the mild base. The proton donor and acceptor molecules form an ion pair prior to photoexcitation. Upon photoexcitation solvent rearrangement occurs on a 1 ps time-scale. The PT reaction time constant is ?2 ps for all three bases. A second decay component of about 10 ps is attributed to the proton transfer in a contact pair bridged by one water molecule. The longest decay component is due to both the excited-state proton transfer (ESPT) to the solvent and the diffusion-assisted PT process between a photoacid and a base pair positioned remotely from each other prior to photoexcitation. PMID:21630647

Presiado, Itay; Gepshtein, Rinat; Erez, Yuval; Huppert, Dan

2011-07-01

200

Dissociative excitation of carbon disulfide by electron impact  

International Nuclear Information System (INIS)

Dissociative excitation of carbon disulfide (CS2) producing CS fragments in the A1? excited state has been studied by optical excitation method in the electron energy range from threshold to 30 eV. The threshold energy for the dissociative excitation has been determined to be 9.33±0.06 eV. It has been also verified that the vibrational population of the CS radicals is inverted for v=0 and 1 levels

201

Vibronic effects on the low-lying electronic excitations in N2O induced by electron impact  

Science.gov (United States)

We report a theoretical study on the valence-shell electronic excitations of N2O induced by electron impact. Momentum transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles have been calculated for the low-lying electronic excitations using theoretical wave functions at the equation-of-motion coupled-cluster singles and doubles level. In the calculation, Herzberg-Teller vibronic effects are taken into account. The computed results are in overall agreement with experimental GOS profiles reported in the literature and reveal prominent roles of the bending vibration of N2O in the B1? and C1? transitions.

Watanabe, Noboru; Takahashi, Masahiko

2014-08-01

202

Electron-impact excitation of Fe II  

International Nuclear Information System (INIS)

We report in this paper the computation of accurate total collision strengths and effective collision strengths for electron-impact excitation of FeII, using the parallel R-matrix program PRMAT. Target states corresponding to the 3d64s, 3d7, 3d64p and 3d54s4s basis configurations were included in the calculations giving rise to a 113 LS state 354 coupled channel problem. Following a detailed systematic study of correlation effects in both the target state and collision wavefunctions, it was found that an additional 21 configuration functions needed to be included in the Configuration Interaction expansion to obtain significantly more accurate target states and collision wavefunctions. This much improved 26-configuration model has been used to calculate converged total effective collision strengths for all sextet to quartet transitions among these levels with total spin S = 2, giving a total of 1785 lines. These calculations have laid the foundation for an approach which may be adopted in the study of electron collisions with the low ionization stages of other iron peak elements. The work has been further extended with the commencement of a Breit-Pauli relativistic calculation for one of the smaller models and includes 262 fine-structure levels and over 1800 coupled channels. At the same time the PRMAT parallel R-matrix package is being extended to include relativistic effects which will allow us to attempt the more sophisticill allow us to attempt the more sophisticated 26-configuration model and produce for the first time the amount and quality of atomic data required to perform a meaningful synthesis of the Fe II spectrum

203

Theory of nuclear excitation by electron capture for heavy ions  

OpenAIRE

The resonant process of nuclear excitation by electron capture (NEEC) in collisions involving highly-charged ions has been investigated theoretically. NEEC is a rare recombination process in which a free electron is captured into a bound shell of an ion with the simultaneous excitation of the nucleus. Total cross sections for NEEC followed by the radiative decay of the excited nucleus are presented for various collision systems. The possibility to observe the NEEC in scattering experiments wi...

Gagyi-palffy, Adriana

2006-01-01

204

Theory of nuclear excitation by electron capture for heavy ions  

OpenAIRE

The resonant process of nuclear excitation by electron capture (NEEC) in collisions involving highly-charged ions has been investigated theoretically. NEEC is a rare recombination process in which a free electron is captured into a bound shell of an ion with the simultaneous excitation of the nucleus. Total cross sections for NEEC followed by the radiative decay of the excited nucleus are presented for various collision systems. The possibility to observe the NEEC in scattering experiments wi...

Gagyi-palffy, Adriana

2007-01-01

205

Matrix photochemistry of small molecules: Influencing reaction dynamics on electronically excited hypersurfaces  

International Nuclear Information System (INIS)

Investigations of chemical reactions on electronically excited reaction surfaces are presented. The role of excited-surface multiplicity is of particular interest, as are chemical reactivity and energy transfer in systems in which photochemistry is initiated through a metal atom ''sensitizer.'' Two approaches are employed: A heavy-atom matrix affords access to forbidden triplet reaction surfaces, eliminating the need for a potentially reactive sensitizer. Later, the role of the metal atom in the photosensitization process is examined directly

206

Modeling coherent excitation energy transfer in photosynthetic light harvesting systems  

Science.gov (United States)

Recent non-linear spectroscopy experiments suggest the excitation energy transfer in some biological light harvesting systems initially occurs coherently. Treating such processes brings significant challenge for conventional theoretical tools that usually involve different approximations. In this dissertation, the recently developed Iterative Linearized Density Matrix (ILDM) propagation scheme, which is non-perturbative and non-Markovian is extended to study coherent excitation energy transfer in various light harvesting complexes. It is demonstrated that the ILDM approach can successfully describe the coherent beating of the site populations on model systems and gives quantitative agreement with both experimental results and the results of other theoretical methods have been developed recently to going beyond the usual approximations, thus providing a new reliable theoretical tool to study this phenomenon. This approach is used to investigate the excited energy transfer dynamics in various experimentally studied bacteria light harvesting complexes, such as Fenna-Matthews-Olsen (FMO) complex, Phycocyanin 645 (PC645). In these model calculations, quantitative agreement is found between computed de-coherence times and quantum beating pattens observed in the non-linear spectroscopy. As a result of these studies, it is concluded that the stochastic resonance behavior is important in determining the optimal throughput. To begin addressing possible mechanics for observed long de-coherence time, various models which include correlation between site energy fluctuations as well as correlation between site energy and inter-site coupling are developed. The influence of both types of correlation on the coherence and transfer rate is explored using with a two state system-bath hamiltonian parametrized to model the reaction center of Rhodobacter sphaeroides bacteria. To overcome the disadvantages of a fully reduced approach or a full propagation method, a brownian dynamics method is developed that only treats those degrees of freedom which directly couple to quantum systems and hence maintain the non-Markovian nature of the dynamics in contrast to the fully reduced master equation. This brownian dynamics approach has a significant reduced computational cost compared with ILDM propagation. The algorithm is tested with a spin-boson-global bath model, and shows potential for capturing non-markovian features that will be lost with a fully reduced master equation description of the excitation energy transfer.

Huo, Pengfei

2011-12-01

207

Electron transfer kinetics at single nanoparticles  

OpenAIRE

The understanding and control of charge transfer at the very smallest scale is fundamental to nanoscience for applications such as catalysis and energy storage. However, the quantitative measurement of the kinetics of electron transfer at the nanoscale at individual free nanoparticles has not hitherto been possible. Here we describe experiments to unambiguously determine the electron transfer kinetics for the reduction of protons at single gold and silver nanoparticles of radii 7-15 nm. We sh...

Kahk, Jm; Rees, Nv; Pillay, J.; Tshikhudo, R.; Vilakazi, S.; Compton, Rg

2012-01-01

208

K- and L-shell resonant transfer and excitation in ion-atom collisions  

International Nuclear Information System (INIS)

Recent experimental studies of resonant transfer and excitation (RTE) in ion-atom collisions are reviewed. In the RTE process correlated electron capture and projectile excitation occur together in a single encounter with a target atom. Measurements of Ca/sup q+/ + H2 (q = 10 to 19) from 100 to 370 MeV establish the projectile charge-state dependence of K-shell RTE and provide a detailed test of the theory. Structure due to RTE is observed in the energy dependence of the total electron-capture cross sections for this collision system. A comparison of the Ca17+ + H2 data with previous results for Ca17+ + He demonstrates the effect of the target-electron momentum distribution on the RTE process. Studies of 230 to 610 MeV Nb31+ + H2 provide information about RTE involving agreement with theoretical calculations. 15 refs., 4 figs

209

On the use of analytical approximate expressions for the transfer rate in excitation transfer kinetics  

International Nuclear Information System (INIS)

We present a discussion about the range of validity of the usual approximate transfer rate expressions used in the description of the kinetics of diffusion-modulated excitation transfer, for a reactive interaction of exponential functional form. We simulate the features of energy transfer by a numerical inversion of the exact Laplace transform of the transfer rate. It is shown that for high diffusion coefficients of the order of 10-5cm2s-1, the kinetics may be well reproduced, even at short times, by the asymptotic form of the transfer rate. For slow molecular displacements, the short time static regime is brought to direct observation, but the transfer rate approaches is asymptotic value at a much later time

210

Excitation curve for auger decay of the (1S2S2P2)3D state of C4+ formed through RTE [Resonant Transfer and Excitation  

International Nuclear Information System (INIS)

The ORNL EN tandem supplied beams of 3.5--9 MeV C3+ ions which were incident on a differentially pumped He target. Auger electrons emitted by fast C4+ ions emerged at 10 deg(lab)w.r.t. the incident ion beam. They were detected using a two stage spectrometer with kinematic refocusing. An excitation curve was obtained for the (1s2s2p)3D state Auger decay to the ground state of C3+. The excitation curve peaks at ?6 MeV, which strongly supports the notion that the state is formed through Resonant Transfer and Excitation. 10 refs., 2 figs

211

Low-energy electron attachment and detachment in vibrationally excited oxygen  

International Nuclear Information System (INIS)

Three-body electron attachment to O2 molecules and electron detachment from O2- ions have been theoretically studied in vibrationally excited oxygen and O2-containing mixtures. Assuming that electron attachment and detachment proceed via the formation of vibrationally excited temporary O2- ions, the rates of these processes were determined on the basis of the statistical approach for the vibrational transfer and relaxation in collisions between O2- ions and O2 molecules. The calculated attachment and detachment rate constants turned out to agree well with available measurements in unexcited oxygen. This method was extended to calculate attachment and detachment rates in vibrationally excited oxygen. It was shown that the effect of vibrational excitation on electron detachment is profound, whereas attachment of low-energy electrons to vibrationally excited O2 is inefficient. The calculated vibrational distribution of stable O2- ions turned out to be non-equilibrium in an excited gas and the effective vibrational temperature of the ions was much lower than the vibrational temperature of molecules. An analytical method was suggested to determine this distribution and the effective vibrational temperature. The calculated rate constants were used to simulate the formation and decay of an electron-beam-generated plasma in N2 : Ogenerated plasma in N2 : O2 mixtures at elevated vibrational temperatures. The calculations showed that vibrational excitation of molecules leads to orders of magnitude increase in the plasma density and in the plasma lifetime, in agreement with available observations.

212

Coherent spin-transfer dynamics in diluted magnetic semiconductor quantum wells even after optical excitation with zero net angular momentum  

Science.gov (United States)

A quantum kinetic study of correlated spin transfer between optically excited electrons and Mn atoms in a ZnMnSe quantum well is presented. The simulations predict genuine signatures of non-Markovian spin dynamics which are particularly pronounced for special two-color laser excitations with a zero net angular momentum where a Markovian theory predicts an almost zero total electron spin for all times. In contrast, in the quantum kinetic simulations a sizable total electron spin builds up. Subsequently, a coherent oscillatory exchange of spin between the electron and Mn subsystems is observed.

Thurn, C.; Cygorek, M.; Axt, V. M.; Kuhn, T.

2013-10-01

213

Two-photon Induced Hot Electron Transfer to a Single Molecule in a Scanning Tunneling Microscope  

OpenAIRE

The junction of a scanning tunneling microscope (STM) operating in the tunneling regime was irradiated with femtosecond laser pulses. A photo-excited hot electron in the STM tip resonantly tunnels into an excited state of a single molecule on the surface, converting it from the neutral to the anion. The electron transfer rate depends quadratically on the incident laser power, suggesting a two-photon excitation process. This nonlinear optical process is further confirmed by t...

Wu, Shiwei; Ho, Wilson

2010-01-01

214

TDDFT study on the excited-state proton transfer of 8-hydroxyquinoline: Key role of the excited-state hydrogen-bond strengthening.  

Science.gov (United States)

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been employed to study the excited-state intramolecular proton transfer (ESIPT) reaction of 8-hydroxyquinoline (8HQ). Infrared spectra of 8HQ in both the ground and the lowest singlet excited states have been calculated, revealing a red-shift of the hydroxyl group (-OH) stretching band in the excited state. Hence, the intramolecular hydrogen bond (O-H···N) in 8HQ would be significantly strengthened upon photo-excitation to the S1 state. As the intramolecular proton-transfer reaction occurs through hydrogen bonding, the ESIPT reaction of 8HQ is effectively facilitated by strengthening of the electronic excited-state hydrogen bond (O-H···N). As a result, the intramolecular proton-transfer reaction would occur on an ultrafast timescale with a negligible barrier in the calculated potential energy curve for the ESIPT reaction. Therefore, although the intramolecular proton-transfer reaction is not favorable in the ground state, the ESIPT process is feasible in the excited state. Finally, we have identified that radiationless deactivation via internal conversion (IC) becomes the main dissipative channel for 8HQ by analyzing the energy gaps between the S1 and S0 states for the enol and keto forms. PMID:25554951

Lan, Sheng-Cheng; Liu, Yu-Hui

2015-03-15

215

Photonic switching of photoinduced electron transfer in a dithienylethene-porphyrin-fullerene triad molecule.  

Science.gov (United States)

A dithienylethene (DTE)-porphyrin (P)-fullerene (C(60)) triad molecule in which intramolecular photoinduced electron transfer is controlled by the photochromic DTE moiety has been prepared. Irradiation of the molecule with visible light gives the open form of the dithienylethene (DTEo). Excitation of the porphyrin gives DTEo-(1)P-C(60), which undergoes photoinduced electron transfer with a time constant of 25 ps to generate DTEo-P(.+)-C(60)(.-). Irradiation with ultraviolet light produces the closed form of the dithienylethene (DTEc). Excitation of DTEc-P-C(60) yields DTEc-(1)P-C(60), whose porphyrin first excited singlet state is quenched in 2.3 ps by singlet-singlet energy transfer to DTEc, generating (1)DTEc-P-C(60) and precluding significant photoinduced electron transfer. Such highly reversible photonically controlled intramolecular photoinduced electron transfer may eventually be useful in the design of photonic or optoelectronic devices. PMID:12083915

Liddell, Paul A; Kodis, Gerdenis; Moore, Ana L; Moore, Thomas A; Gust, Devens

2002-07-01

216

Vibration-assisted resonance in photosynthetic excitation energy transfer  

CERN Document Server

Coherent quantum energy transfer, as observed in photosynthetic pigment-protein complexes, is inhibited by energetic disorder. While this difficulty can be overcome to some extent by the addition of environmental noise, it has recently has begun to be appreciated that discrete intra- and/or intermolecular vibrational modes may play an important role in quantum dynamics. We present a microscopic mechanism by which intramolecular vibrational modes create resonant energy transfer pathways, enhancing the efficiency of both coherent and dephasing-assisted transfer. The principles of this vibration-assisted resonance are illustrated in a simple model based on one energy-transfer branch of the well-characterised Fenna-Matthews-Olson complex. Despite its simplicity, this model captures the interplay between strong electronic coupling that produces delocalised exciton states and resonance-enhanced weak coupling to local vibrational modes. Analytical and numerical results show that intramolecular vibrations can enhance...

Irish, E K; Lovett, B W

2013-01-01

217

Erbium emission in MOS light emitting devices: from energy transfer to direct impact excitation.  

Science.gov (United States)

The electroluminescence (EL) at 1.54 ?m of metal–oxide–semiconductor (MOS) devices withEr3C ions embedded in the silicon-rich silicon oxide (SRSO) layer has been investigated under different polarization conditions and compared with that of erbium doped SiO2 layers. EL time-resolved measurements allowed us to distinguish between two different excitation mechanisms responsible for the Er3C emission under an alternate pulsed voltage signal (APV). Energy transfer from silicon nanoclusters (Si-ncs) to Er3C is clearly observed at low-field APV excitation. We demonstrate that sequential electron and hole injection at the edges of the pulses creates excited states in Si-ncs which upon recombination transfer their energy to Er3C ions. On the contrary, direct impact excitation of Er3C by hot injected carriers starts at the Fowler–Nordheim injection threshold (above 5 MV cm(-1)) and dominates for high-field APV excitation. PMID:22414783

Ramí?rez, J M; Ferrarese Lupi, F; Jambois, O; Berencén, Y; Navarro-Urrios, D; Anopchenko, A; Marconi, A; Prtljaga, N; Tengattini, A; Pavesi, L; Colonna, J P; Fedeli, J M; Garrido, B

2012-03-30

218

Transfer excitation reactions in fast proton-helium collisions  

CERN Document Server

Continuing previous work, we have measured the projectile scattering-angle dependency for transfer excitation of fast protons (300-1200 keV/u) colliding with helium (p+He $\\rightarrow$ H + He$^{+ *}$). Our high-resolution fully differential data are accompanied by calculations, performed in the plane wave first Born approximation and the eikonal wave Born approximation. Experimentally we find a deep minimum in the differential cross section around 0.5 $mrad$. The comparison with our calculations shows that describing the scattering angle dependence of transfer exitation in fast collisions requires to go beyond the first Born approximation and in addition to use initial state wave function, which contains some degree of angular correlations.

Schöffler, M S; Chuluunbaatar, O; Houamer, S; Galstyan, A G; Titze, J N; Jahnke, T; Schmidt, L Ph H; Schmidt-B"ocking, H; D"orner, R; Popov, Yu V; Bulychev, A A

2013-01-01

219

Long-distance fluorescence quenching by electron transfer in rigid solutions  

International Nuclear Information System (INIS)

Fluorescence of aromatic molecules in solid solutions at room temperature and low temperature is quenched by addition of strong electron donors. The efficiency of quenching correlates well with calculated energetics for electron transfer (ET) from the added donors to the excited molecules, D + A* ? D+ + A-. The quencher (D) always had a substantially higher-lying excited state than did A to prevent electronic energy transfer (e.g., Foerster transfer). The A* fluorescence decreased exponentially with D concentration, being halved by addition of about 0.1 M D in cases with favorable energetics for electron transfer. These quenching measurements are interpreted as indicating electron transfer over distances up to 15 A (center to center). These maximum quenching distances are in excellent agreement with electron tunneling distances measured for ion-molecule reactions by pulse radiolysis. Formation of donor-acceptor complexes during sample preparation is a possible alternate interpretattion, but several features of the data provide evidence against this alternative

220

Search for excited electrons using the ZEUS detector  

International Nuclear Information System (INIS)

This paper reports a search for excited electrons at the HERA electron-proton collider. In a sample corresponding to an integrated luminosity of 26 nb-1, no evidence was found for any resonant state decaying into e-?, ?W- or e-Z0. Limits on the coupling strength of an excited electron have been determined for masses between 45 and 225 GeV. This study also reports the observation of the wide-angle e? Compton scattering process. (orig.)

221

Minimal excitation states of electrons in one-dimensional wires  

CERN Document Server

A strategy is proposed to excite particles from a Fermi sea in a noise-free fashion by electromagnetic pulses with realistic parameters. We show that by using quantized pulses of simple form one can suppress the particle-hole pairs which are created by a generic excitation. The resulting many-body states are characterized by one or several particles excited above the Fermi surface accompanied by no disturbance below it. These excitations carry charge which is integer for noninteracting electron gas and fractional for Luttinger liquid. The operator algebra describing these excitations is derived, and a method of their detection which relies on noise measurement is proposed.

Keeling, J; Levitov, L S

2006-01-01

222

Minimal excitation states of electrons in one-dimensional wires.  

Science.gov (United States)

A strategy is proposed to excite particles from a Fermi sea in a noise-free fashion by electromagnetic pulses with realistic parameters. We show that by using quantized pulses of simple form one can suppress the particle-hole pairs which are created by a generic excitation. The resulting many-body states are characterized by one or several particles excited above the Fermi surface accompanied by no disturbance below it. These excitations carry charge which is integer for noninteracting electron gas and fractional for Luttinger liquid. The operator algebra describing these excitations is derived, and a method of their detection which relies on noise measurement is proposed. PMID:17025911

Keeling, J; Klich, I; Levitov, L S

2006-09-15

223

Vibrational excitation induced proton transfer in hydrated nafion membranes.  

Science.gov (United States)

We study the energy relaxation and structural relaxation dynamics of hydrated protons in Nafion membranes at different hydration levels using femtosecond infrared transient absorption spectroscopy. At low hydration levels we observe that the excitation of the proton vibration of an Eigen-like proton hydration structure leads to a structural relaxation process in which the Eigen-like structure evolves to a Zundel-like proton hydration structure. This reorganization leads to a transfer of the proton charge and closely follows the mechanism of infrared-induced adiabatic proton transfer that has been proposed by S. Hammes-Schiffer, J. T. Hynes, and others. At high hydration levels, the spectral dynamics are dominated by vibrational energy relaxation and subsequent cooling of the proton hydration structures and the surrounding water molecules. Using a kinetic analysis of the transient spectral data, we determine the rates of proton transfer, vibrational energy relaxation, and cooling as a function of hydration level. We find that infrared-induced proton transfer occurs at all hydration levels but becomes less observable at high hydration levels due to the increasingly dominant influence of the vibrational energy relaxation. PMID:25506744

Liu, Liyuan; Bakker, Huib J

2015-02-12

224

Theoretical investigation of excited state proton transfer process in the N-salicylidene-2-bromoethylamine.  

Science.gov (United States)

Excited state reaction coordinate and the consequent energy profiles of a new Schiff base, N-salicylidene-2-bromoethylamine, have been investigated at the CC2 level of theory. The electron-driven proton transfer and torsional deformation have been identified as the most important photochemical reaction coordinates. In contrast to the ground state, the excited state potential energy profile shows a barrierless dissociation pattern along the O-H stretching coordinate, which verifies the proton transfer reaction along the O-H coordinate at the S(1) state. The calculations showed that the PT is electron driven and that the S(1) transition has charge transfer character. The keto-type S(1) state attained by barrierless proton transfer is found to be unstable via a torsional motion, which provides fast access to a S(1)-S(0) conical intersection. From the conical intersection, a barrierless reaction path directs the system back to the enol-type minimum of the S(0) potential energy surface, thus closing the photocycle. PMID:23312032

Moghadam, Ahmad J; Omidyan, Reza; Mirkhani, Valiollah; Azimi, Gholamhasan

2013-01-31

225

Excited State Proton Transfer Dynamics of Topotecan Inside Biomimicking Nanocavity.  

Science.gov (United States)

The excited state proton transfer (ESPT) dynamics of a potentially important anticancer drug, Topotecan (TPT), has been explored in aqueous reverse micelle (RM) using steady-state and time-resolved fluorescence measurements. Both the time-resolved emission spectrum and time-resolved area normalized emission spectrum infer the generation of excited state zwitterionic form of TPT from the excited state cationic form of TPT, as a result of ESPT process from the -OH group of TPT to the nearby water molecule. The ESPT dynamics were found to be severely retarded inside the nanocavities of RMs, yielding time constants of 250 ps to 1.0 ns, which is significantly slower than the dynamics obtained in bulk water (32 ps). The observed slow ESPT dynamics in RM compared to bulk water is mainly attributed to the sluggish hydrogen-bonded network dynamics of water molecules inside the nanocavity of RM and the screening of the sodium ions present at the interface. PMID:25105353

Koninti, Raj Kumar; Gavvala, Krishna; Sengupta, Abhigyan; Hazra, Partha

2014-08-19

226

Explicit calculation of the excited electronic states of the photosystem II reaction centre.  

Science.gov (United States)

The excited states of sets of the cofactors found in the photosystem II reaction centre have been calculated directly as a multi-monomer supermolecule for the first time. Time-dependent density functional theory was used with the CAM-B3LYP functional. Multiple excited states for each cofactor were found at lower energies than the lowest energy state corresponding to charge transfer states (in which an electron is shifted from one cofactor to another). The electrostatic environment was found to have a dramatic impact on the excited state energies, with the effect of a surrounding dielectric medium being less significant. PMID:25523136

Frankcombe, Terry J

2015-02-01

227

Electron collisions and internal excitation in stored molecular ion beams  

OpenAIRE

In storage ring experiments the role, which the initial internal excitation of a molecular ion can play in electron collisions, and the effect of these collisions on the internal excitation are investigated. Dissociative recombination (DR) and inelastic and super-elastic collisions are studied in the system of He2+. The DR rate coefficient at low energies depends strongly on the initial vibrational excitation in this system. Therefore changes in the DR rate coefficient are a very sensitive...

Buhr, Henrik

2006-01-01

228

Transport coefficients in nonequilibrium gas-mixture flows with electronic excitation.  

Science.gov (United States)

In the present paper, a one-temperature model of transport properties in chemically nonequilibrium neutral gas-mixture flows with electronic excitation is developed. The closed set of governing equations for the macroscopic parameters taking into account electronic degrees of freedom of both molecules and atoms is derived using the generalized Chapman-Enskog method. The transport algorithms for the calculation of the thermal-conductivity, diffusion, and viscosity coefficients are proposed. The developed theoretical model is applied for the calculation of the transport coefficients in the electronically excited N/N(2) mixture. The specific heats and transport coefficients are calculated in the temperature range 50-50,000 K. Two sets of data for the collision integrals are applied for the calculations. An important contribution of the excited electronic states to the heat transfer is shown. The Prandtl number of atomic species is found to be substantially nonconstant. PMID:19905461

Kustova, E V; Puzyreva, L A

2009-10-01

229

Electron donor-acceptor quenching and photoinduced electron transfer for coumarin dyes  

Science.gov (United States)

The fluorescence of 7-aminocoumarins is quenched by a variety of organic electron donors or acceptors in acetonitrile. In general, donors with half-wave oxidation potentials less positive than 1.0 V vs SCE and acceptors with reduction potentials less negative than -1.5 V vs SCE are candidates for diffusion limited quenching of coumarin singlet states. Profiles of quenching rates are consistent with calculated free energies for electron transfer between excited coumarins and donors or acceptors. In flash photolysis experiments electron transfer for several dyes and quenchers (e.g., methyl viologen) is demonstrated. Relatively low yields of net electron transfer are consistently obtained due to inefficient ionic photodissociation via singlet quenching or a low yield of more photoactive coumarin triplets. Electrochemical properties of the coumarins have been investigated by cyclic voltammetry with the indications of reversible oxidation and irreversible reduction as important processes.

Jones, G., II; Griffin, S. F.; Choi, C. Y.; Bergmark, W. R.

1983-10-01

230

Hierarchical control of electron-transfer  

DEFF Research Database (Denmark)

In this chapter the role of electron transfer in determining the behaviour of the ATP synthesising enzyme in E. coli is analysed. It is concluded that the latter enzyme lacks control because of special properties of the electron transfer components. These properties range from absence of a strong back pressure by the protonmotive force on the rate of electron transfer to hierarchical regulation of the expression of the gens that encode the electron transfer proteins as a response to changes in the bioenergetic properties of the cell.The discussion uses Hierarchical Control Analysis as a paradigm. This allows one to analyse a complex system of regulatory interactions in terms of the importance of the contributing factors.

Westerhoff, Hans V.; Jensen, Peter Ruhdal

1997-01-01

231

Ultrafast coherent dynamics of nonadiabatically coupled quasi-degenerate excited states in molecules: Population and vibrational coherence transfers  

International Nuclear Information System (INIS)

Graphical abstract: Temporal behaviors due to quantum mechanical interferences between the nonadiabatically coupled quasi-degenerate excited states (b and c) after a coherent excitation. Highlights: ? A nonadiabatic theory of quasi-degenerate ?-electronic states in aromatic molecules. ? Quantum interferences between the nonadiabatically-coupled ?-electronic states. ? Analysis of time-dependent vibrational coherence transfer via nonadiabatic couplings. - Abstract: Results of a theoretical study of ultrafast coherent dynamics of nonadiabatically coupled quasi-degenerate ?-electronic excited states of molecules were presented. Analytical expressions for temporal behaviors of population and vibrational coherence were derived using a simplified model to clarify the quantum mechanical interferences between the two coherently excited electronic states, which appeared in the nuclear wavepacket simulations [M. Kanno, H. Kono, Y. Fujimura, S.H. Lin, Phys. Rev. Lett 104 (2010) 108302]. The photon-polarization direction of the linearly polarized laser, which controls the populations of the two quasi-degenerate electronic states, determines constructive or destructive interference. Features of the vibrational coherence transfer between the two coupled quasi-electronic states through nonadiabatic couplings are also presented. Information on both the transition frequency and nonadiabatic coupling matrix element between the two states can be obtained by analyzing signals of obtained by analyzing signals of two kinds of quantum beats before and after transfer through nonadiabatic coupling.

232

Auger-assisted electron transfer from photoexcited semiconductor quantum dots.  

Science.gov (United States)

Although quantum confined nanomaterials, such as quantum dots (QDs) have emerged as a new class of light harvesting and charge separation materials for solar energy conversion, theoretical models for describing photoinduced charge transfer from these materials remain unclear. In this paper, we show that the rate of photoinduced electron transfer from QDs (CdS, CdSe, and CdTe) to molecular acceptors (anthraquinone, methylviologen, and methylene blue) increases at decreasing QD size (and increasing driving force), showing a lack of Marcus inverted regime behavior over an apparent driving force range of ?0-1.3 V. We account for this unusual driving force dependence by proposing an Auger-assisted electron transfer model in which the transfer of the electron can be coupled to the excitation of the hole, circumventing the unfavorable Franck-Condon overlap in the Marcus inverted regime. This model is supported by computational studies of electron transfer and trapping processes in model QD-acceptor complexes. PMID:24359156

Zhu, Haiming; Yang, Ye; Hyeon-Deuk, Kim; Califano, Marco; Song, Nianhui; Wang, Youwei; Zhang, Wenqing; Prezhdo, Oleg V; Lian, Tianquan

2014-03-12

233

Advances in electron transfer chemistry, v.6  

CERN Document Server

It is clear that electron transfer chemisty is now one of the most active areas of chemical study. Advances in Electron Transfer Chemistry has been designed to allow scientists who are developing new knowledge in this rapidly expanding area to describe their most recent research findings. This volume will serve those interested in learning about current breakthroughs in this rapidly expanding area of chemical research.

Mariano, PS

1999-01-01

234

Coupled electron transfers in artificial photosynthesis  

OpenAIRE

Light-induced charge separation in molecular assemblies has been widely investigated in the context of artificial photosynthesis. Important progress has been made in the fundamental understanding of electron and energy transfer and in stabilizing charge separation by multi-step electron transfer. In the Swedish Consortium for Artificial Photosynthesis, we build on principles from the natural enzyme photosystem II and Fe-hydrogenases. An important theme in this biomimetic effort is that of cou...

Hammarstro?m, Leif; Styring, Stenbjörn

2008-01-01

235

Photophysical and electron transfer studies of a stable carbocation  

Science.gov (United States)

Photophysical and electron transfer properties of the stable trioxatriangulenium carbocation ( 1) are reported. Photophysical studies include absorption, fluorescence and phosphorescence spectra, singlet and triplet state quantum yields and lifetimes. Both the singlet and triplet excited states of 1 can accept an electron from donor molecules leading to the formation of the donor radical cation and the radical of 1. In aqueous solution, 1 can photo-oxidize DNA nucleosides such as guanosine and adenosine indicating that 1 may have potential use as a DNA cleaving agent.

Dileesh, S.; Gopidas, K. R.

2000-11-01

236

Efficient laser desorption ionization mass spectrometry of polycyclic aromatic hydrocarbons using excitation energy transfer from anthracene  

International Nuclear Information System (INIS)

Highlights: ? Femtomolar detection of PAHs such as perylene and benzopyrene was achieved. ? Photoexcited anthracene molecules transferred their energy to PAHs. ? Electronically excited PAHs were then excited to be ions. ? Two-photon ionization process was necessary to complete the ionization process. ? The number of defect sites could be reduced by the annealing procedure. - Abstract: Polycyclic aromatic hydrocarbons (PAHs), such as perylene and benzopyrene, doped at amounts on the order of femtomol (?10?15 mol) in anthracene crystals could be detected by laser desorption ionization mass spectrometry. Sensitivity was roughly 103 times higher than that of LDI method in our experimental conditions. It was revealed from the excitation power dependence of the peak intensity of PAHs on the mass spectra that two-photon excitation in one UV pulse was necessary to complete the ionization process. It was also clarified that the number of defect sites that trap excitons generated in anthracene crystals could be reduced by the annealing procedure, by which an efficient energy transfer between anthracene and PAHs became possible

237

Efficient laser desorption ionization mass spectrometry of polycyclic aromatic hydrocarbons using excitation energy transfer from anthracene  

Energy Technology Data Exchange (ETDEWEB)

Highlights: ? Femtomolar detection of PAHs such as perylene and benzopyrene was achieved. ? Photoexcited anthracene molecules transferred their energy to PAHs. ? Electronically excited PAHs were then excited to be ions. ? Two-photon ionization process was necessary to complete the ionization process. ? The number of defect sites could be reduced by the annealing procedure. - Abstract: Polycyclic aromatic hydrocarbons (PAHs), such as perylene and benzopyrene, doped at amounts on the order of femtomol (?10{sup ?15} mol) in anthracene crystals could be detected by laser desorption ionization mass spectrometry. Sensitivity was roughly 10{sup 3} times higher than that of LDI method in our experimental conditions. It was revealed from the excitation power dependence of the peak intensity of PAHs on the mass spectra that two-photon excitation in one UV pulse was necessary to complete the ionization process. It was also clarified that the number of defect sites that trap excitons generated in anthracene crystals could be reduced by the annealing procedure, by which an efficient energy transfer between anthracene and PAHs became possible.

Fujimori, Kensuke [Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397 (Japan); Fujino, Tatsuya, E-mail: fujino@tmu.ac.jp [Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397 (Japan)

2013-06-20

238

Elementary Electronic Excitations in Graphene Nanoribbons  

CERN Document Server

We analyze the collective mode spectrum of graphene nanoribbons within the random phase approximation. In the undoped case, only metallic armchair nanoribbons support a propagating plasmon mode. Landau damping of this mode is shown to be suppressed through the chirality of the single particle wavefunctions. We argue that undoped zigzag nanoribbons should not support plasmon excitations because of a broad continuum of particle-hole excitations associated with surface states, into which collective modes may decay. Doped nanoribbons have properties similar to those of semiconductor nanowires, including a plasmon mode dispersing as $q\\sqrt{-\\ln qW}$ and a static dielectric response that is divergent at $q=2k_F$.

Brey, L

2007-01-01

239

Dynamics of ground and excited state vibrational relaxation and energy transfer in transition metal carbonyls.  

Science.gov (United States)

Nonlinear vibrational spectroscopy provides insights into the dynamics of vibrational energy transfer in and between molecules, a crucial phenomenon in condensed phase physics, chemistry, and biology. Here we use frequency-domain 2-dimensional infrared (2DIR) spectroscopy to investigate the vibrational relaxation (VR) and vibrational energy transfer (VET) rates in different solvents in both the electronic ground and excited states of Re(Cl)(CO)3(4,4'-diethylester-2,2'-bipyridine), a prototypical transition metal carbonyl complex. The strong C?O and ester C?O stretch infrared reporters, located on opposite sides of the molecule, were monitored in the 1600-2100 cm(-1) spectral region. VR in the lowest charge transfer triplet excited state ((3)CT) is found to be up to eight times faster than in the ground state. In the ground state, intramolecular anharmonic coupling may be solvent-assisted through solvent-induced frequency and charge fluctuations, and as such VR rates are solvent-dependent. In contrast, VR rates in the solvated (3)CT state are surprisingly solvent-insensitive, which suggests that predominantly intramolecular effects are responsible for the rapid vibrational deactivation. The increased VR rates in the excited state are discussed in terms of intramolecular electrostatic interactions helping overcome structural and thermodynamic barriers for this process in the vicinity of the central heavy atom, a feature which may be of significance to nonequilibrium photoinduced processes observed in transition metal complexes in general. PMID:25198700

Delor, Milan; Sazanovich, Igor V; Towrie, Michael; Spall, Steven J; Keane, Theo; Blake, Alexander J; Wilson, Claire; Meijer, Anthony J H M; Weinstein, Julia A

2014-10-01

240

Excitation energy transfer in the peridinin-chlorophyll a-protein complex modeled using configuration interaction.  

Science.gov (United States)

We modeled excitation energy transfer (EET) in the peridinin-chlorophyll a-protein (PCP) complex of dinoflagellate Amphidinium carterae to determine which pathways contribute dominantly to the high efficiency of this process. We used complete active space configuration interaction (CAS-CI) to calculate electronic structure properties of the peridinin (PID) and chlorophyll a (CLA) pigments in PCP and the transition density cube (TDC) method to calculate Coulombic couplings between energy transfer donors and acceptors. Our calculations show that the S1 ? Qy EET pathway from peridinin to chlorophyll a is the dominant energy transfer pathway in PCP, with two sets of interactions-between PID612 and CLA601 and between PID622 and CLA602-contributing most strongly. EET lifetimes for these two interactions were calculated to be 2.66 and 2.90, with quantum efficiencies of 85.75 and 84.65%, respectively. The calculated Coulombic couplings for EET between two peridinin molecules in the strongly allowed S2 excited states are extremely large and suggest excitonic coupling between pairs of peridinin S2 states. This methodology is also broadly applicable to the study of EET in other photosynthetic complexes and/or organic photovoltaics, where both single and double excitations are present and donor and acceptor molecules are tightly packed. PMID:25007401

Bricker, William P; Lo, Cynthia S

2014-08-01

241

Heat Transfer Augmentation for Electronic Cooling  

Directory of Open Access Journals (Sweden)

Full Text Available Problem statement: The performance of electronic devices has been improving along with the rapid technology development. Cooling of electronic systems is consequently essential in controlling the component temperature and avoiding any hot spot. The study aims to review the present electronic cooling methods which are widely used in electronic devices. Approach: There are several methods to cool down the electronics components such as the pin-fin heat sink, confined jet impingement, heat pipe, micro heat sink and so on. Results: The cooling techniques can obviously increase heat transfer rate. Nonetheless, for active and passive cooling methods the pressure drop could extremely rise, when the heat transfer rate is increased. Conclusion: When the cooling techniques are used, it is clearly seen that the heat transfer increases with pressure drop. To avoid excessive expense due to high pressure drop, optimization method is required to obtain optimum cost and cooling rate.

Suabsakul Gururatana

2012-01-01

242

Collisional excitation of electron Landau levels in strong magnetic fields  

Science.gov (United States)

The cross sections for the excitation and deexcitation of the quantized transverse energy levels of an electron in a magnetic field are calculated for electron-proton and electron-electron collisions in light of the importance of the cross sections for studies of X-ray pulsar emission. First-order matrix elements are calculated using the Dirac theory of the electron, thus taking into account relativistic effects, which are believed to be important in accreting neutron stars. Results for the collisional excitation of ground state electrons by protons are presented which demonstrate the importance of proton recoil and relativistic effects, and it is shown that electron-electron excitations may contribute 10 to 20% of the excitation rate from electron-proton scattering in a Maxwellian plasma. Finally, calculations of the cross section for electron-proton small-angle scattering are presented which lead to relaxation rates for the electron velocity distribution which are modified by the magnetic field, and to a possible increase in the value of the Coulomb logarithm.

Langer, S. H.

1981-01-01

243

A stochastic reorganizational bath model for electronic energy transfer  

CERN Document Server

The fluctuations of optical gap induced by the environment play crucial roles in electronic energy transfer dynamics. One of the simplest approaches to incorporate such fluctuations in energy transfer dynamics is the well known Haken-Strobl-Reineker model, in which the energy-gap fluctuation is approximated as a white noise. Recently, several groups have employed molecular dynamics simulations and excited-state calculations in conjunction to take the thermal fluctuation of excitation energies into account. Here, we discuss a rigorous connection between the stochastic and the atomistic bath models. If the phonon bath is treated classically, time evolution of the exciton-phonon system can be described by Ehrenfest dynamics. To establish the relationship between the stochastic and atomistic bath models, we employ a projection operator technique to derive the generalized Langevin equations for the energy-gap fluctuations. The stochastic bath model can be obtained as an approximation of the atomistic Ehrenfest equ...

Fujita, Takatoshi; Aspuru-Guzik, Alan

2014-01-01

244

Describing Excited State Intramolecular Proton Transfer in Dual Emissive Systems: A Density Functional Theory Based Analysis.  

Science.gov (United States)

The excited state intramolecular proton transfer (ESIPT) reaction taking place within 2-(2-hydroxyphenyl)benzoxazole (HBT) and two recently experimentally characterized napthalimide derivatives-known as N-1 and N-4-has been investigated in order to identify and test a possible protocol for the description and complete mechanistic and electronic characterization of the reaction at the excited state. This protocol is based on density functional theory, time-dependent density functional theory, and a recently proposed electron density based index (DCT). This method is able to identify all stable species involved in the reaction, discriminate between possible reaction pathways over potential energy surfaces (PES), which are intrinsically very flat and difficult to characterize, and quantitatively measure the excited state charge transfer character throughout the reaction. The photophysical properties of the molecules (i.e., absorption and emission wavelength) are also quantitatively determined via the implicit inclusion of solvent effects in the case of toluene and, the more polar, tetrahydrofuran. The accuracy obtained with this protocol then opens up the possibility of the ab initio design of molecules exhibiting ESIPT for tailored applications such as highly selective molecular sensors. PMID:25208048

Wilbraham, Liam; Savarese, Marika; Rega, Nadia; Adamo, Carlo; Ciofini, Ilaria

2014-10-01

245

Study of quasi one electron systems: excitation of the active electron to its first excited state  

International Nuclear Information System (INIS)

Experimental angular differential cross sections (ADCS) of four Quasi-One-Electron (QOE) scattering systems (Li+, Na+ + H, and Be+, Mg+ + He) are reported. These ADCS are determined for exciting the active electron to its first excited state. The angular range of the ACS is from 0 to ?3 mrad/sub lab/ with an angular resolution of ?120 ?rad/sub lab/. The energy range is from 19.4 to 150 keV with an energy-loss resolution of ?1 eV. The ADCS are compared to the available theories. The agreement between experiment and theory ranges from very poor to fair. No experiments exist which allow direct comparison to the present results. Integration of the present ADCS gives results which are compared to the experimental and theoretical total He targets cross section results. The agreement changes from energy to energy. Theoretical total cross section results exist in the cases involving H targets are in very poor to fair agreement with the present results. No experimental total cross sections exist in the H target case for comparison with the present results. The most striking feature occurs in the Na+ + H ADCS. Here the value of the ADCS at zero angle changes by 2 1/2 orders of magnitude from (3.96 +/- 2.00) x 10-15 cm2/sr at 35.9 keV (v = 1/4 a.u.) to (1.03 +/- 0.25) x 10-12 cm2/sr at 143.8 keV (v = 1/2 a.u.)

246

Vibration-assisted resonance in photosynthetic excitation-energy transfer  

Science.gov (United States)

Understanding how the effectiveness of natural photosynthetic energy-harvesting systems arises from the interplay between quantum coherence and environmental noise represents a significant challenge for quantum theory. Recently it has begun to be appreciated that discrete molecular vibrational modes may play an important role in the dynamics of such systems. Here we present a microscopic mechanism by which intramolecular vibrations may be able to contribute to the efficiency and directionality of energy transfer. Excited vibrational states create resonant pathways through the system, supporting fast and efficient energy transport. Vibrational damping together with the natural downhill arrangement of molecular energy levels gives intrinsic directionality to the energy flow. Analytical and numerical results demonstrate a significant enhancement of the efficiency and directionality of energy transport that can be directly related to the existence of resonances between vibrational and excitonic levels.

Irish, E. K.; Gómez-Bombarelli, R.; Lovett, B. W.

2014-07-01

247

Energy transfer from the host excitations to Ce3+ ions in scandium borate  

International Nuclear Information System (INIS)

Efficient energy transfer from the host excitations (self-trapped excitons, STE) excited at ??160 nm to Ce3+ impurity ions was observed in scandium borate (ScBO3). The luminescence and excitation spectra as well as time-resolved emission data were obtained. Energy transfer rates from the STE to Ce3+ of about 108 s-1 are observed and these are within an order of magnitude of estimates based on dipole-dipole energy transfer

248

Convection heat transfer in electronic equipment cooling  

Science.gov (United States)

To maintain the best possible thermal environment in electronic packages, the engineer must establish the most efficient path for heat transfer from the electronic devices to an external cooling agent. The path is typically subdivided into internal and external components, representing, respectively, heat transfer by conduction through different materials and interfaces separating the devices from the package surface and heat transfer by convection from the surface to the coolant. Depending on the scale and speed of the electronic circuits, as well as on constraints imposed by nonthermal considerations, the coolant may be a gas or a liquid and heat transfer may be by natural, forced, or mixed convection or, in the case of a liquid, by pool or forced convection boiling. In this paper a comprehensive review of convection cooling options is provided.

Incropera, F. P.

1988-11-01

249

Electron impact excitation of the gas-phase tellurium  

International Nuclear Information System (INIS)

The excitation processes of vapor tellurium by low energy electron impact (0-50 eV) were studied. The emission spectra in the wavelength range from 200 to 525 nm for electron-impact energies of 10, 15, 20, 30 eV were investigated. The optical excitation functions of the most intense atomic lines and molecular bands were measured. It was found that in the temperature range of 270-320°C a lot of diatomic molecules of Te2 along with polyatomic tellurium molecules were in the vapor. Atomic lines of Te* formed through dissociative excitation of tellurium molecules. (authors)

250

Reaction dynamics of electronically excited alkali atoms with simpler molecules  

International Nuclear Information System (INIS)

The reactions of electronically excited sodium atoms with simple molecules have been studied in crossed molecular beams experiments. Electronically excited Na(32P/sub 3/2/, 42D/sub 5/2/, and 52S/sub 1/2/) were produced by optical pumping using single frequency dye lasers. The effects of the symmetry, and the orientation and alignment of the excited orbital on the chemical reactivity, and detailed information on the reaction dynamics were derived from measurements of the product angular and velocity distributions. 12 refs., 9 figs

251

Analyses of donor-acceptor distance-dependent rates of photo-induced electron transfer in flavoproteins with three kinds of electron transfer theories  

International Nuclear Information System (INIS)

Reported donor-acceptor distance-dependent rates of photo-induced electron transfer from tryptophan (Trp), tyrosine (Tyr), and benzoate (Bz) to the excited isoalloxazine in ten flavoprotein systems were analyzed with three kinds of electron transfer theories by Marcus, by Bixon and Jortner, and also by Kakitani, Yoshimori, and Mataga. Average donor-acceptor distances that were obtained from X-ray structures of flavoproteins were used for the analysis, rather than the edge-to-edge distance. The observed photo-induced electron transfer rates were best reproduced by the Kakitani, Yoshimori, and Mataga theory

252

Coaxial electron gun. Working principles and excitation processes in helium  

International Nuclear Information System (INIS)

The coaxial electron gun has been used for investigation of excitation of n=2 levels of helium plasma with a monokinetic electron beam. The electron distribution function by energy was calculated for a wide range of current and pressure alterations, allowing to make comparison between calculated and measured values of n=2 levels populations. The saturation of all states' populations (except 23S at a current increase above 1.5-2 A) showed the importance of inelastic collisions with electrons

253

Nuclear excitation by electronic transition - NEET  

International Nuclear Information System (INIS)

Experiments seeking to demonstrate nuclear excitation induced by synchrotron radiation have been enabled by the development of intense synchrotron radiation. The phenomena has been demonstrated in 197Au, while realistic upper limits for 189Os have been established. These new experiments report probabilities for NEET which are orders of magnitude below earlier experiments. A new experiment to measure atomic-nuclear mixing in 189Os is described. The experimental claim of NEET in isomeric 178Hf is not credible

254

Spectral analysis of electron transfer kinetics II  

CERN Document Server

Electron transfer processes in Debye solvents are studied using a spectral analysis method recently proposed. Spectral structure of a nonadiabatic two-state diffusion equation is investigated to reveal various kinetic regimes characterized by a broad range of physical parameters; electronic coupling, energy bias, reorganization energy, and solvent relaxation rate. Within this unified framework, several kinetic behaviors of the electron transfer kinetics, including adiabatic Rabi oscillation, crossover from the nonadiabatic to adiabatic limits, transition from the incoherent to coherent kinetic limits, and dynamic bath effect, are demonstrated and compared with results from previous theoretical models. Dynamics of the electron transfer system is also calculated with the spectral analysis method. It is pointed out that in the large reorganization energy case the nonadiabatic diffusion equation exhibits a non-physical behavior, yielding a negative eigenvalue.

Jung, Y J; Jung, YounJoon; Cao, Jianshu

2002-01-01

255

Direct conversion of graphite into diamond through electronic excited states  

CERN Document Server

An ab initio total energy calculation has been performed for electronic excited states in diamond and rhombohedral graphite by the full-potential linearized augmented plane wave method within the framework of the local density approximation (LDA). First, calculations for the core-excited state in diamond have been performed to show that the ab initio calculations based on the LDA describe the wavefunctions in the electronic excited states as well as in the ground state quite well. Fairly good coincidence with both experimental data and theoretical prediction has been obtained for the lattice relaxation of the core exciton state. The results of the core exciton state are compared with nitrogen-doped diamond. Next, the structural stability of rhombohedral graphite has been investigated to examine the possibility of the transition into the diamond structure through electronic excited states. While maintaining the rhombohedral symmetry, rhombohedral graphite can be spontaneously transformed to cubic diamond. Tota...

Nakayama, H

2003-01-01

256

Locally-Excited (LE) versus Charge-Transfer (CT) Excited State Competition in a Series of Para-Substituted Neutral Green Fluorescent Protein (GFP) Chromophore Models.  

Science.gov (United States)

In this paper, I provide a characterization of the low-energy electronic structure of a series of para-substituted neutral green fluorescent protein (GFP) chromophore models using a theoretical approach that blends linear free energy relationships (LFERs) with state-averaged complete-active-space self-consistent field (SA-CASSCF) theory. The substituents are chosen to sample the Hammett ?p scale from R = F to NH2, and a model of the neutral GFP chromophore structure (R = OH) is included. I analyze the electronic structure for different members of the series in a common complete-active-space valence-bond (CASVB) representation, exploiting an isolobal analogy between active-space orbitals for different members of the series. I find that the electronic structure of the lowest adiabatic excited state is a strong mixture of weakly coupled states with charge-transfer (CT) or locally excited (LE) character and that the dominant character changes as the series is traversed. Chromophores with strongly electron-donating substituents have a CT-like excited state such as expected for a push-pull polyene or asymmetric cyanine. Chromophores with weakly electron-donating (or electron-withdrawing) substituents have an LE-like excited state with an ionic biradicaloid structure localized to the ground-state bridge ? bond. PMID:25343562

Olsen, Seth

2014-11-01

257

Collisional electron transfer to photoexcited acceptor radical anions  

DEFF Research Database (Denmark)

In this article, we show that photoexcitation of radical anions facilitates electron transfer from sodium atoms in femtosecond encounters. Thus, excitation of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and fluorinated TCNQ (TCNQ-F(4)) anions to the second optically active state at 478 nm led to increases in the yields of dianions of about 20% and 10%, respectively. Photoexcitation with a nanosecond-long laser pulse was done a few microseconds before the ions entered the sodium collision cell so that none of the ions would be in any of the initially reached doublet-excited states. We suggest an explanation for the higher electron capture cross section based on the formation of long-lived quartet state anions. Excitation of TCNQ anions within the lowest-energy absorption band, where there are no accessible quartet states, led instead to a lower yield of dianions. There are at least three explanations for the lower dianion yields: (1) Depletion of the monoanion beam due to photodetachment after the absorption ofminimum two photons; (2) Formation of short-lived vibrationally excited dianions that decay by electron autodetachment prior to identification; and (3) Lower electron capture cross sections of vibrationally excited monoanions. Similar losses in dianion signal can occur at 478 nm so the actual yield of dianions at this wavelength due to the population of quartet states is therefore greater than that observed. Our methodology devises a more efficient route for the production of molecular dianions, and at the same time it may provide information on long-lived electronic states.

Wyer, Jean Ann; StØchkel, Kristian

2012-01-01

258

Strong electronic excitation effect of swift heavy ions on C60 films  

International Nuclear Information System (INIS)

Strong electronic excitation effect of C60 films induced by 2.0 GeV 136Xe and 2.7 GeV 238U ions was investigated by means of Fourier transform infrared (FTIR). Raman Scattering and X-ray diffraction (XRD) spectroscopies. A new peak located at 670 cm-1, which corresponds to an unknown structure, was observed in the FTIR spectra of C60 films irradiated for the first time. The variation in intensity of 670 cm-1 peak with electronic energy loss and irradiation dose were studied. The analysis results indicated that electronic energy transfer dominates the damage process of C60 films. The partial recovery of the damage in irradiated C60 films at middle electronic energy loss is attributed to an annealing effect of the strong electronic excitation. The ion velocity also plays a role in the process of the damage creation

259

Theoretical study of intermolecular energy transfer involving electronically excited molecules: He(/sup 1/S) + H/sub 2/(B /sup 1/. sigma. /sub u//sup +/). [Solution for coupled channel equations  

Energy Technology Data Exchange (ETDEWEB)

To further understanding of gas phase collision dynamics involving electronically-excited molecules, a fully quantum mechanical study of He + H/sub 2/(B /sup 1/..sigma../sub u//sup +/) was undertaken. Iterative natural orbital configuration interaction (CI) calculations were performed to obtain the interaction potential between He and H/sub 2/(B /sup 1/..sigma../sub u//sup +/). The potential energy surface (PES) is highly anisotropic and has a van der Waals well of about 0.03 eV for C/sub 2v/ approach. Avoided PES crossings occur with He + H/sub 2/(E,F /sup 1/..sigma../sub g//sup +/) and with He + H/sub 2/(X /sup 1/..sigma../sub g//sup +/) and cause a local maximum and a deep minimum in the He + H/sub 2/(B /sup 1/..sigma../sub u//sup +/) PES, respectively. The crossing with He + H/sub 2/(X /sup 1/..sigma../sub g//sup +/) provides a mechanism for fluorescence quenching. The computed CI energies were combined with previous multi-reference double excitation CI calculations and fit with analytic functions for convenience in scattering calculations. Accurate dipole polarizabilities and quadrupole moment of H/sub 2/(B /sup 1/..sigma../sub u//sup +/) were computed for use in the multipole expansion, which is the analytic form of the long-range PES. 129 refs., 28 figs., 35 tabs.

Grimes, R.M.

1986-11-01

260

Intramolecular electron transfer in bipyridinium disulfides.  

Science.gov (United States)

Reductive cleavage of disulfide bonds is an important step in many biological and chemical processes. Whether cleavage occurs stepwise or concertedly with electron transfer is of interest. Also of interest is whether the disulfide bond is reduced directly by intermolecular electron transfer from an external reducing agent or mediated intramolecularly by internal electron transfer from another redox-active moiety elsewhere within the molecule. The electrochemical reductions of 4,4'-bipyridyl-3,3'-disulfide (1) and the di-N-methylated derivative (2(2+)) have been studied in acetonitrile. Simulations of the cyclic voltammograms in combination with DFT (density functional theory) computations provide a consistent model of the reductive processes. Compound 1 undergoes reduction directly at the disulfide moiety with a substantially more negative potential for the first electron than for the second electron, resulting in an overall two-electron reduction and rapid cleavage of the S-S bond to form the dithiolate. In contrast, compound 2(2+) is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the disulfide moiety. Most interesting, the second reduction of the bipyridinium moiety results in a fast and reversible intramolecular two-electron transfer to reduce the disulfide moiety and form the dithiolate. Thus, the redox-active bipyridinium moiety provides a low energy pathway for reductive cleavage of the S-S bond that avoids the highly negative potential for the first direct electron reduction. Following the intramolecular two-electron transfer and cleavage of the S-S bond the bipyridinium undergoes two additional reversible reductions at more negative potentials. PMID:24528295

Hall, Gabriel B; Kottani, Rudresha; Felton, Greg A N; Yamamoto, Takuhei; Evans, Dennis H; Glass, Richard S; Lichtenberger, Dennis L

2014-03-12

261

Protein electron transfer: Dynamics and statistics  

Science.gov (United States)

Electron transfer between redox proteins participating in energy chains of biology is required to proceed with high energetic efficiency, minimizing losses of redox energy to heat. Within the standard models of electron transfer, this requirement, combined with the need for unidirectional (preferably activationless) transitions, is translated into the need to minimize the reorganization energy of electron transfer. This design program is, however, unrealistic for proteins whose active sites are typically positioned close to the polar and flexible protein-water interface to allow inter-protein electron tunneling. The high flexibility of the interfacial region makes both the hydration water and the surface protein layer act as highly polar solvents. The reorganization energy, as measured by fluctuations, is not minimized, but rather maximized in this region. Natural systems in fact utilize the broad breadth of interfacial electrostatic fluctuations, but in the ways not anticipated by the standard models based on equilibrium thermodynamics. The combination of the broad spectrum of static fluctuations with their dispersive dynamics offers the mechanism of dynamical freezing (ergodicity breaking) of subsets of nuclear modes on the time of reaction/residence of the electron at a redox cofactor. The separation of time-scales of nuclear modes coupled to electron transfer allows dynamical freezing. In particular, the separation between the relaxation time of electro-elastic fluctuations of the interface and the time of conformational transitions of the protein caused by changing redox state results in dynamical freezing of the latter for sufficiently fast electron transfer. The observable consequence of this dynamical freezing is significantly different reorganization energies describing the curvature at the bottom of electron-transfer free energy surfaces (large) and the distance between their minima (Stokes shift, small). The ratio of the two reorganization energies establishes the parameter by which the energetic efficiency of protein electron transfer is increased relative to the standard expectations, thus minimizing losses of energy to heat. Energetically efficient electron transfer occurs in a chain of conformationally quenched cofactors and is characterized by flattened free energy surfaces, reminiscent of the flat and rugged landscape at the stability basin of a folded protein.

Matyushov, Dmitry V.

2013-07-01

262

Ultrafast electron diffraction studies of optically excited thin bismuth films  

Energy Technology Data Exchange (ETDEWEB)

This thesis contains work on the design and the realization of an experimental setup capable of providing sub-picosecond electron pulses for ultrafast electron diffraction experiments, and performing the study of ultrafast dynamics in bismuth after optical excitation using this setup. (orig.)

Rajkovic, Ivan

2008-10-21

263

Ultrafast electron diffraction studies of optically excited thin bismuth films  

International Nuclear Information System (INIS)

This thesis contains work on the design and the realization of an experimental setup capable of providing sub-picosecond electron pulses for ultrafast electron diffraction experiments, and performing the study of ultrafast dynamics in bismuth after optical excitation using this setup. (orig.)

264

Electron-impact excitation of ions  

International Nuclear Information System (INIS)

A review of electron-ion beam experiments is given. Techniques, difficulties, and present trends in this area are discussed. Measured cross sections are compared with theoretical results and the current level of agreement is assessed. 74 references

265

Electronic excitation in metals through hyperthermal atoms  

Energy Technology Data Exchange (ETDEWEB)

A hyperthermal hydrogen/deuterium atom beam source with a defined energy distribution has been employed to investigate the kinetically induced electron emission from noble metal surfaces. A monotonous increase in the emission yield was found for energies between 15 and 200 eV. This, along with an observed isotope effect, is described in terms of a model based on Boltzmann type electron energy distributions.

Kovacs, D A [Institute for Applied Plasma Physics, CPST, Ruhr-Universitaet Bochum, 44780 Bochum (Germany); Babkina, T [Institute for Plasma and Atomic Physics, CPST, Ruhr-Universitaet Bochum, 44780 Bochum (Germany); Gans, T [Institute for Plasma and Atomic Physics, CPST, Ruhr-Universitaet Bochum, 44780 Bochum (Germany); Czarnetzki, U [Institute for Plasma and Atomic Physics, CPST, Ruhr-Universitaet Bochum, 44780 Bochum (Germany); Diesing, D [Institute of Physical and Theoretical Chemistry, Universitaet Duisburg-Essen, 45141 Essen (Germany)

2006-12-21

266

Electronic excitation in metals through hyperthermal atoms  

International Nuclear Information System (INIS)

A hyperthermal hydrogen/deuterium atom beam source with a defined energy distribution has been employed to investigate the kinetically induced electron emission from noble metal surfaces. A monotonous increase in the emission yield was found for energies between 15 and 200 eV. This, along with an observed isotope effect, is described in terms of a model based on Boltzmann type electron energy distributions

267

Electronic Excitations in Lanthanum Cuprates Measured by Resonant Inelastic X-ray Scattering  

Science.gov (United States)

Excitations of the valence electrons in the high-temperature superconducting cuprate La2--xSr xCuO4 were measured by Resonant Inelastic X-ray Scattering (RIXS). Several types of electronic excitations resonant at the Cu 1s?4p transition were studied over a wide range of dopings 0 ? x ? 0.35. A 500 meV excitation was observed at a reduced momentum transfer q=(pi 0) corresponding to the zone boundary, whose temperature and doping dependence was the same as the two-magnon Raman scattering mode. The momentum dependence of this 2-magnon excitation agrees with recent theoretical calculations. Momentum resolved measurements of the x=0 sample revealed a broad range of excitations above and below the main charge transfer peak, and their dispersions were measured across the Brillouin zone. These include a dispersionless ˜1.8 eV peak, which is either a local crystal field ( d-d excitation) or dipole-forbidden charge transfer excitation, and a dispersive 2.2 eV peak identified as a Zhang-Ng type charge-transfer exciton. The 2.2 eV peak was less dispersive than predicted from the theoretical models, due to electron-phonon coupling, as illustrated by the temperature dependent shift in the peak position. With increased hole doping, the RIXS spectral weight transfers from higher to lower energies, analogous to earlier optical conductivity studies. At the finite momentum of q=(pi 0), however, the changes are most systematic: an isosbestic point was observed at 2.2 eV where the spectra of all dopings cross, and spectral weight is transferred from high to low energies, with near-linear dependence on x, in a manner suggesting that the integrated RIXS intensity is preserved. The intensity and energy variations of the spectral peaks, as well as the isosbestic point and possible sum rule, could be explained qualitatively by a rigid three-band model which includes the non-bonding oxygen, upper Hubbard, and Zhang-Rice singlet bands. The estimated properties of the bands, such as width and energy separation, are in reasonably quantitative agreement with current theoretical models and angle-resolved photoemission measurements. Moreover, anomalies in the doping dependence are similar to those observed in other types of spectroscopies. These results underscore the relevance of the RIXS method in understanding the electronic behavior of the Lanthanum cuprates.

Ellis, David Shai

268

Photoinduced electron transfer in ordered polymers  

Energy Technology Data Exchange (ETDEWEB)

Long range photoinduced electron transfer between electron donor and acceptor groups is of considerable current interest in terms of strategies for artificial photosynthesis and studies regarding the redox properties of proteins. As part of an extensive study of long range electron transfer involving biopolymers, we have carried out this year investigations of the assembly of electron transfer agents in a system of model short peptides. Also studied is a polyelectrolyte that can adopt a helical conformation when electrostatically complexed with organic dye counter-ions. The principal interest in these systems has to do with the well ordered secondary structures adopted by peptide polymers, and the capabilities for synthetic modification of peptide side chains and end groups with chromophores or electroactive substituents. The present report gives a brief account of the following elements of work related to photochemical electron transfer themes: (1) the synthesis and photochemical characterization of chromophore-bound peptides and amino acid model compounds based on the amino acids, tryptophan and the spacer residue, alanine (Ala); (2) the study of binding of the cationic organic dye to a peptide electrolyte, for which cooperative dye loading and helix formation is important; and (3) completion of the synthesis of a new series of acridinium chromophores that have rod-like'' arrangements of inked aryl rings for assembly of electron donor-acceptor systems that will exhibit especially long lived charge separation.

Jones, G. II.

1991-12-01

269

Electron transfer dissociation of oligonucleotide cations  

Science.gov (United States)

Electron transfer dissociation (ETD) of multi-protonated 6-20-mer oligonucleotides and 12- and 14-mer duplexes is compared to collision activated dissociation (CAD). ETD causes efficient charge reduction of the multi-protonated oligonucleotides in addition to limited backbone cleavages to yield sequence ions of low abundance. Subsequent CAD of the charge-reduced oligonucleotides formed upon electron transfer, in a net process termed electron transfer collision activated dissociation (ETcaD), results in rich fragmentation in terms of w, a, z, and d products, with a marked decrease in the abundance of base loss ions and internal fragments. Complete sequencing was possible for nearly all oligonucleotides studied. ETcaD of an oligonucleotide duplex resulted in specific backbone cleavages, with conservation of weaker non-covalent bonds.

Smith, Suncerae I.; Brodbelt, Jennifer S.

2009-06-01

270

Resonant excitation of molecules by low-energy electrons  

Directory of Open Access Journals (Sweden)

Full Text Available Low-energy electron impact vibrational and electronic excitation cross sections of the CO, N2 and CO2 molecules are measured by use of a high resolution crossed-beams double trochoidal electron spectrometer. The spectrometer is designed to work in standard and time-of-flight regimes. The energy dependences of the resonant vibrational excitation of the first several vibrational levels of the N2, CO, and CO2 molecules, have been measured. Characteristic substructures in energy excitation spectra in the cases of N2 and CO have been obtained and discussed for some vibrational channels for the first time. The ratio of forward-to-backward scattered electrons from the 2? resonance in CO is found to be equal to 1, and thus the angular distribution of scattered electrons to be symmetric relative to 90°. This conclusion supports the fact that the contribution of the p? partial wave is dominant in the energy region of the 2? resonance in CO. The energy dependences of the near threshold resonant excitation of the valence and Rydberg states of the N2 and CO molecules have been measured. The cross sections of the near threshold resonant excitation of the C 3?u valence state, and the E 3?+ g and a'' 1?+ g Rydberg states of the N2 molecule have been measured. In the case of the CO molecule, the cross sections of the near threshold resonant excitation of the a 3? valence state, and the b 3?+ and B 1?+ Rydberg states have been measured. Resonant structures in excitation functions of all measured electronic states are observed and their locations are compared with resonances obtained in different decay channels.

Popari? Goran B.

2008-01-01

271

Spatial distribution of electron densities during optical excitation of C{sub 60}  

Energy Technology Data Exchange (ETDEWEB)

We calculate the spatial distribution of the oscillating electron density for the low energy optical excitations in C{sub 60}, using ab initio time-dependent density functional theory. We contrast this with the unscreened free response of the Kohn-Sham eigenfunctions, in which the charge transfer is between hemispheres and the simple density oscillation patterns are very similar for electrons outside and inside the C{sub 60} radius. In the screened case, the oscillation patterns involve charge transfer on the scale of interatomic distances. The exterior and interior densities play different roles in the oscillator strength and screening. Almost all of the dipole oscillation for optical absorption strength is from exterior electrons, a consequence of the superposition of {pi} and {sigma} electron oscillations. The lowest transition with appreciable strength is dominated by screening from {pi} electrons.

Hess, Bret C; Jensen, Daniel S; Okhrimenko, Ivan G, E-mail: bret_hess@byu.ed [Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602 (United States)

2010-10-10

272

Directed emission of fast electrons from excited metal nanoparticles  

Energy Technology Data Exchange (ETDEWEB)

Experimental studies on laser excited silver particles reveal a strong asymmetric electron emission characteristic giving electron kinetic energies of up to 1.3 keV in the direction of the laser polarization axis. It could be shown that in a dual pulse experiment the evolution of the plasmon frequency following the pump pulse enables resonant excitation by the probe pulse. The resulting electron energies, gained in the nanometer scaled microplasma, as well as the spatial asymmetry in emission direction cannot be explained by a conventional hydrodynamical or a purely coulomb driven expansion process. We present a novel acceleration mechanism through plasmon assisted, phase-matched electron-cluster rescattering, which is capable to explain the generation of a directed emission of high energy electrons obtained in the experiment. Moreover, Vlasov simulations show that an attosecond electron pulse train is generated.

Passig, Johannes; Fennel, Thomas; Truong, Nguyen Xuan; Tiggesbaeumker, Josef; Meiwes-Broer, Karl-Heinz [Universitaet Rostock (Germany). Institut fuer Physik

2008-07-01

273

Rate coefficients for electron impact excitation of CO  

International Nuclear Information System (INIS)

Highlights: • Rate coefficients for electron impact excitation of CO have been calculated. • All relevant non-elastic processes have been included in calculations. • Equilibrium and non-equilibrium conditions have been considered. • Electron energy distribution functions were determined by a Monte Carlo simulation. • Rate coefficients have also been determined in the presence of magnetic field. - Abstract: Rate coefficients for electron impact excitation of the CO molecule have been calculated both for equilibrium and non-equilibrium conditions in the presence of electric and magnetic fields. The rate coefficients have been determined for all relevant non-elastic processes: rotational excitation, vibrational excitation, electronic excitation into singlet and triplet states and particularly for ionization. In the case of non-equilibrium conditions, we had to determine electron energy distribution functions needed for rate coefficients calculations. The distribution functions were obtained by employing a Monte Carlo simulation developed in our laboratory. The simulations were performed for moderate values of electric field over gas number density ratios, E/N, from 0 to 1000 Td. Also, the rate coefficients have been determined in presence of magnetic field for typical values of magnetic field over gas number density ratios, B/N, from 500 to 3000 Hx. The results of equilibrium rate coefficients along with non-equilibrium ones have been shown

274

Rate coefficients for electron impact excitation of CO  

Energy Technology Data Exchange (ETDEWEB)

Highlights: • Rate coefficients for electron impact excitation of CO have been calculated. • All relevant non-elastic processes have been included in calculations. • Equilibrium and non-equilibrium conditions have been considered. • Electron energy distribution functions were determined by a Monte Carlo simulation. • Rate coefficients have also been determined in the presence of magnetic field. - Abstract: Rate coefficients for electron impact excitation of the CO molecule have been calculated both for equilibrium and non-equilibrium conditions in the presence of electric and magnetic fields. The rate coefficients have been determined for all relevant non-elastic processes: rotational excitation, vibrational excitation, electronic excitation into singlet and triplet states and particularly for ionization. In the case of non-equilibrium conditions, we had to determine electron energy distribution functions needed for rate coefficients calculations. The distribution functions were obtained by employing a Monte Carlo simulation developed in our laboratory. The simulations were performed for moderate values of electric field over gas number density ratios, E/N, from 0 to 1000 Td. Also, the rate coefficients have been determined in presence of magnetic field for typical values of magnetic field over gas number density ratios, B/N, from 500 to 3000 Hx. The results of equilibrium rate coefficients along with non-equilibrium ones have been shown.

Vojnovi?, M.; Popovi?, M. [University of Belgrade, Faculty of Physics, Studentski Trg 12, P.O. Box 44, 11000 Belgrade (Serbia); Risti?, M.M. [University of Belgrade, Faculty of Physical Chemistry, Studentski Trg 12, P.O. Box 137, 11000 Belgrade (Serbia); Vi?i?, M.D. [University of Belgrade, Faculty of Physics, Studentski Trg 12, P.O. Box 44, 11000 Belgrade (Serbia); Popari?, G.B., E-mail: goran_poparic@ff.bg.ac.rs [University of Belgrade, Faculty of Physics, Studentski Trg 12, P.O. Box 44, 11000 Belgrade (Serbia)

2013-09-23

275

Review of biorthogonal coupled cluster representations for electronic excitation  

CERN Document Server

Single reference coupled-cluster (CC) methods for electronic excitation are based on a biorthogonal representation (bCC) of the (shifted) Hamiltonian in terms of excited CC states, also referred to as correlated excited (CE) states, and an associated set of states biorthogonal to the CE states, the latter being essentially configuration interaction (CI) configurations. The bCC representation generates a non-hermitian secular matrix, the eigenvalues representing excitation energies, while the corresponding spectral intensities are to be derived from both the left and right eigenvectors. Using the perspective of the bCC representation, a systematic and comprehensive analysis of the excited-state CC methods is given, extending and generalizing previous such studies. Here, the essential topics are the truncation error characteristics and the separability properties, the latter being crucial for designing size-consistent approximation schemes. Based on the general order relations for the bCC secular matrix and the...

Schirmer, J

2009-01-01

276

Dissociation of vibrationally excited D2+ by electrons  

OpenAIRE

Abstract We have investigated the electron impact dissociation of the deuterium molecular cation assuming a Franck-Condon distribution of the initial vibrational states given by electron impact ionization of D2. The electron energy range 1-12 eV has been considered. The relative contributions of the initial vibrational states to the total cross sections of the D2+ dissociative recombination and excitation have been determined.

Fifirig, Magda; Stroe, Marius

2011-01-01

277

Experimental investigation of excitation of argon metastables by an electron swarm in argon nitrogen gas mixtures  

International Nuclear Information System (INIS)

Excitation of argon metastable levels (43P2+43P0) in a nonselfsustained discharge in Ar(>=99%)+N2(2 second positive group which is the result of excitation transfer from Ar metastables to N2 molecules. The method is based on the assumption, experimentally verified, that addition of small quantities of N2 do not change markedly the probability of Ar excitation. The obtained excitation coefficients for production of Arsup(m) in the E/N range (20-70)x10-21 Vm2 are (3.5-13)x10-22 m2 with an estimated uncertainty of +-50%. Simultaneously, excitation coefficients for direct excitation of nitrogen C-state by swarm electrons in Ar-N2 mixtures have been obtained. A comparison of nitrogen spectra excited by electrons and by argon metastables showed significant differences in relative population of vibrational and rotational levels of the N2(C3PIsub(u)) state in these two cases. (orig.)

278

Isomer triggering via nuclear excitation by electron capture  

OpenAIRE

Triggering of long-lived nuclear isomeric states via coupling to the atomic shells in the process of nuclear excitation by electron capture (NEEC) is studied. NEEC occurring in highly-charged ions can excite the isomeric state to a triggering level that subsequently decays to the ground state. We present total cross sections for NEEC isomer triggering considering experimentally confirmed low-lying triggering levels and reaction rates based on realistic experimental parameter...

Pa?lffy, Adriana; Evers, Jo?rg; Keitel, Christoph H.

2007-01-01

279

Resonant Raman scattering by elementary electronic excitations in semiconductor structures  

OpenAIRE

We explain quantitatively why resonant Raman scattering spectroscopy, an extensively used experimental tool in studying elementary electronic excitations in doped low dimensional semiconductor nanostructures, always produces an observable peak at the so-called "single particle" excitation although the standard theory predicts that there should be no such single particle peak in the Raman spectra. We have thus resolved an experimental puzzle which dates back more than twenty-...

Sarma, S. Das; Wang, Daw-wei

1999-01-01

280

An Exciting Aspect of Nanotechnology: Unimolecular Electronics  

Directory of Open Access Journals (Sweden)

Full Text Available This is a brief update on our experimental work towards better one-molecule-thick monolayer rectifiers of electrical current, and on theoretical progress towards a one-molecule amplifier of electrical current. This program aims to provide electronic devices at the 2 to 3 nm level, as a dramatic advance towards practical integrated circuits of the future.

Metzger R. M.

2013-08-01

281

Collective two-particle excitations in cuprates and manganites: Resonant inelastic x-ray scattering and electron energy loss spectroscopy  

International Nuclear Information System (INIS)

The two-particle charge excitations of Sr2CuO3, which contains one-dimensional corner-sharing CuO2-chains, were studied by Resonant Inelastic X-ray Scattering (RIXS) at the Cu K-edge. At the center of the Brillouin zone, the dependence of the various charge-transfer excitations on the incident photon energy Ei was studied in detail. The different charge transfer excitations resonate for different intermediate states, i.e., different Ei, which allows to draw conclusions about the symmetry of the created excitations. Further, the RIXS results are discussed in comparison to previous Electron Energy Loss Spectroscopy (EELS) studies. A similar approach was then used to investigate the two-particle excitations (La,Sr)2MnO4. Surprisingly, the excitations observed for the doped manganite materials share similar traits with the ones observed for the doped cuprates.

282

Photophysics, excited-state double-proton transfer and hydrogen-bonding properties of 5-deazaalloxazines.  

Science.gov (United States)

The photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S0 -Si transitions were investigated using the ab initio methods [MP2, CIS(D), EOM-CCSD] with the correlation-consistent basis sets. Also the time-dependent density functional theory (TD-DFT) has been employed. The lowest singlet excited states of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine are predicted to have the ?, ?* character, whereas similar alloxazines have two close-lying ?, ?* and n, ?* transitions. Experimental steady-state and time-resolved spectral studies indicate formation of an isoalloxazinic excited state via excited-state double-proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5-deazaalloxazine molecule. Solvatochromism of both 5-deazaalloxazine and its 1,3-dimethyl substituted derivative was analyzed using the Kamlet-Taft scale and four-parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute-solvent hydrogen-bonding interactions in their excited state. PMID:24816028

Prukala, Dorota; Khmelinskii, Igor; Koput, Jacek; Gierszewski, Mateusz; P?dzi?ski, Tomasz; Sikorski, Marek

2014-01-01

283

Measuring excitation-energy transfer with a real-time time-dependent density functional theory approach  

CERN Document Server

We investigate the time an electronic excitation travels in a supermolecular setup using a measurement process in an open quantum-system framework. The approach is based on the stochastic Schr\\"odinger equation and uses a Hamiltonian from time-dependent density functional theory (TDDFT). It treats electronic-structure properties and intermolecular coupling on the level of TDDFT, while it opens a route to the description of dissipation and relaxation via a bath operator that couples to the dipole moment of the density. Within our study, we find that in supermolecular setups small deviations of the electronic structure from the perfectly resonant case have only minor influence on the pathways of excitation-energy transfer, thus lead to similar transfer times. Yet, sizable defects cause notable slowdown of the energy spread.

Hofmann, Dirk; Di Ventra, Massimiliano; Kümmel, Stephan

2012-01-01

284

Facile Interfacial Electron Transfer of Hemoglobin  

Directory of Open Access Journals (Sweden)

Full Text Available Abstract: We herein describe a method of depositing hemoglobin (Hb and sulfonated polyaniline (SPAN on GC electrodes that facilitate interfacial protein electron transfer. Well-defined, reproducible, chemically reversible peaks of Hb and SPAN can be obtained in our experiments. We also observed enhanced peroxidase activity of Hb in SPAN films. These results clearly showed that SPAN worked as molecular wires and effectively exchanged electrons between Hb and electrodes.Mediated by Conjugated Polymers

Chunhai Fan

2005-12-01

285

Cycloreversion of ?-lactams via photoinduced electron transfer.  

Science.gov (United States)

The radical anions of ?-lactams, photogenerated in the presence of DABCO as an electron donor, undergo cycloreversion via N-C4 bond cleavage, back electron transfer and final C2-C3 bond cleavage, leading to olefins. The involved intermediates are 1,4-radical anions and 1,4-biradicals. The experimental observations are consistent with the results of DFT calculations. PMID:25223340

Pérez-Ruiz, Raúl; Sáez, Jose A; Jiménez, M Consuelo; Miranda, Miguel A

2014-11-14

286

Vibrational energy transfer in selectively excited diatomic molecules  

International Nuclear Information System (INIS)

Single rovibrational states of HCl(v=2), HBr(v=2), DCl(v=2), and CO(v=2) were excited with a pulsed optical parametric oscillator (OPO). Total vibrational relaxation rates near - resonance quenchers were measured at 2950K using time resolved infrared fluorescence. These rates are attributed primarily to V - V energy transfer, and they generally conform to a simple energy gap law. A small deviation was found for the CO(v) + DCl(v') relaxation rates. Upper limits for the self relaxation by V - R,T of HCl(v=2) and HBr(v=2) and for the two quantum exchange between HCl and HBr were determined. The HF dimer was detected at 2950K and 30 torr HF pressure with an optoacoustic spectrometer using the OPO. Pulsed and chopped, resonant and non-resonant spectrophones are analyzed in detail. From experiments and first order perturbation theory, these V - V exchange rates appear to behave as a first order perturbation in the vibrational coordinates. The rotational dynamics are known to be complicated however, and the coupled rotational - vibrational dynamics were investigated theoreticaly in infinite order by the Dillon and Stephenson and the first Magnus approximations. Large ?J transitions appear to be important, but these calculations differ by orders of magnitude on specific rovibrational transition rates. Integration of the time dependent semiclassical equations by a modified Gordon method and a rotationally distorted wave approximation are discussed as methods which would treat the rotational motion more accurately. 225 references

287

Low-energy charge transfer excitations in NiO  

International Nuclear Information System (INIS)

Comparative analysis of photoluminescence (PL) and photoluminescence excitation (PLE) spectra of NiO poly- and nanocrystals in the spectral range 2-5.5 eV reveals two PLE bands peaked near 3.7 and 4.6 eV with a dramatic rise in the low-temperature PLE spectral weight of the 3.7 eV PLE band in the nanocrystalline NiO as compared with its polycrystalline counterpart. In frames of a cluster model approach we assign the 3.7 eV PLE band to the low-energy bulk-forbidden p-d (t1g(?)-eg) charge transfer (CT) transition which becomes the allowed one in the nanocrystalline state while the 4.6 eV PLE band is related to a bulk allowed d-d (eg-eg) CT transition scarcely susceptible to the nanocrystallization. The PLE spectroscopy of the nanocrystalline materials appears to be a novel informative technique for inspection of different CT transitions.

288

Vibrationally Assisted Electron Transfer Mechanism of Olfaction: Myth or Reality?  

DEFF Research Database (Denmark)

Smell is a vital sense for animals. The mainstream explanation of smell is based on recognition of the odorant molecules through characteristics of their surface, e.g., shape, but certain experiments suggest that such recognition is complemented by recognition of vibrational modes. According to this suggestion an olfactory receptor is activated by electron transfer assisted through odorant vibrational excitation. The hundreds to thousands of different olfactory receptors in an animal recognize odorants over a discriminant landscape with surface properties and vibrational frequencies as the two major dimensions. In the present paper we introduce the vibrationally assisted mechanism of olfaction and demonstrate for several odorants that, indeed, a strong enhancement of an electron tunneling rate due to odorant vibrations can arise. We discuss in this regard the influence of odorant deuteration and explain, thereby, recent experiments performed on Drosophila melanogaster. Our demonstration is based on known physical properties of biological electron transfer and on ab initio calculations on odorants carried out for the purpose of the present study. We identify a range of physical characteristics which olfactory receptors and odorants must obey for the vibrationally assisted electron transfer mechanism to function. We argue that the stated characteristics are feasible for realistic olfactory receptors, noting, though, that the receptor structure presently is still unknown, but can be studied through homology modeling.

Solov'yov, Ilia; Chang, Po-Yao

2012-01-01

289

Role quark degrees of freedom in electron-induced excitation of dibaryon resonances  

International Nuclear Information System (INIS)

The role of quark degrees of freedom in electron-induced excitation of dibaryon resonances (1D2-3F3-1G4) is studied. The electroproduction of dibaryons at 6-quark component 6q (e, e1) 6q* in the deuteron electro disintegration reaction is considered. Electroproduction cross sections of dibaryon resonances within the hybrid model in the region of high (Q2 > or approximately 2-4 GeV2/c2) momentum transfers are evaluated

290

Emission Spectroscopy as a Probe into Photoinduced Intramolecular Electron Transfer in Polyazine Bridged Ru(II,Rh(III Supramolecular Complexes  

Directory of Open Access Journals (Sweden)

Full Text Available Steady-state and time-resolved emission spectroscopy are valuable tools to probe photochemical processes of metal-ligand, coordination complexes. Ru(II polyazine light absorbers are efficient light harvesters absorbing in the UV and visible with emissive 3MLCT excited states known to undergo excited state energy and electron transfer. Changes in emission intensity, energy or band-shape, as well as excited state lifetime, provide insight into excited state dynamics. Photophysical processes such as intramolecular electron transfer between electron donor and electron acceptor sub-units may be investigated using these methods. This review investigates the use of steady-state and time-resolved emission spectroscopy to measure excited state intramolecular electron transfer in polyazine bridged Ru(II,Rh(III supramolecular complexes. Intramolecular electron transfer in these systems provides for conversion of the emissive 3MLCT (metal-to-ligand charge transfer excited state to a non-emissive, but potentially photoreactive, 3MMCT (metal-to-metal charge transfer excited state. The details of the photophysics of Ru(II,Rh(III and Ru(II,Rh(III,Ru(II systems as probed by steady-state and time-resolved emission spectroscopy will be highlighted.

Karen J. Brewer

2010-08-01

291

Electron transfer experiments and atomic magnetism values. Progress report, February 1, 1975--September 30, 1975  

International Nuclear Information System (INIS)

Progress in the first seven months of this new research is described. A new apparatus was constructed, tested and moved to Oak Ridge National Laboratory for studies using the Penning ion source test facility. Preliminary electron transfer cross section results for N4+, N5+, He2+ and C5+ ions on gases were obtained. Energy loss measurements made to date support expectations that single electron transfer for multiply-charged ions colliding with gas atoms produces excited final state ions

292

Electron dynamics and intermolecular energy transfer in aqueous solutions studied by X-ray electron spectroscopy  

Science.gov (United States)

X-ray photoelectron spectroscopy measurements from a vacuum liquid microjet are performed to investigate the electronic structure of aqueous solutions. Here, focus is on the excited-state dynamics of chloride and hydroxide anions in water, following core-level excitation. A series of Cl^-(aq) charge-transfer-to-solvent (CTTS) states, and their ultrafast relaxation, on the time scale of the core hole, is identified from the occurrence of spectator Auger decay. Resonant oxygen 1s excitation of aqueous hydroxide, in contrast, leads to non-local decay, involving energy transfer into a neighboring water molecule. This channel is argued to arise from the weak hydrogen donor bond of OH^-(aq), and thus identifies a special transient hydration configuration, which can explain hydroxide's unusual and fast transport in water. Analogous measurements from pure water point to a similar relaxation channel, which is concluded from a strong isotope effect. The characteristic resonance spectral features are considerably stronger for H2O(aq) than for D2O(aq). As for OH^-(aq) the results can be understood in terms of energy transfer from the excited water molecule to a neighbor water molecule.

Winter, Bernd

2009-03-01

293

Excitation energy transfer between closely spaced multichromophoric systems: Effects of band mixing and intraband relaxation  

OpenAIRE

We theoretically analyze the excitation energy transfer between two closely spaced linear molecular J-aggregates, whose excited states are Frenkel excitons. The aggregate with the higher (lower) exciton band edge energy is considered as the donor (acceptor). The celebrated theory of F\\"orster resonance energy transfer (FRET), which relates the transfer rate to the overlap integral of optical spectra, fails in this situation. We point out that in addition to the well-known fa...

Didraga, C.; Malyshev, V. A.; Knoester, J.

2006-01-01

294

Electron impact excitation of argon in the extreme vacuum ultraviolet. [excitation functions, resonances, cross sections  

Energy Technology Data Exchange (ETDEWEB)

Polarization-free excitation cross sections in the extreme vacuum ultraviolet have been measured for electron impact on Ar. Observed spectral features were those lines of Ar I and Ar II which lie between 700 and 1100 A. Excitation functions were measured for the Ar I resonance line at 1048 A and the Ar II resonance line at 920 A. Peak cross sections for these two lines were found to be (39.4 +- 7.9) x 10/sup -18/ and (6.9 +- 1.4) x 10/sup -18/ cm/sup 2/, respectively. At low energies, excitation of the Ar II resonance line is dominated by an electron exchange transition. (AIP)

Mentall, J.E.; Morgan, H.D.

1976-09-01

295

Theoretical Study on Inner Shell Electron Impact Excitation of Lithium  

International Nuclear Information System (INIS)

Cross sections for electron impact excitation of lithium from the ground state 1s22s to the excited states 1s2s2, 1s2p2, 1s2snp (n = 2–5), 1s2sns (n = 3–5), 1s2pns (n = 3–5), and 1s2pnp (n = 3–5) are calculated by using a full relativistic distorted wave method. The latest experimental electron energy loss spectra for inner-shell electron excitations of lithium at a given incident electron energy of 2500 eV [Chin. Phys. Lett. 25(2008)3649] have been reproduced by the present theoretical investigation excellently. At the same time, the structures of electron energy loss spectra of lithium at low incident electron energy are also predicted theoretically, it is found that the electron energy loss spectra in the energy region of 55–57eV show two-peak structures. (atomic and molecular physics)

296

Investigations of the electron capture excitation in slow ion-atom collisions  

International Nuclear Information System (INIS)

Line radiation between 40 and 110 nm from rare gas ions and hydrogen molecules excited by electron impact was observed with a polarization-free arranged vuv-monochromator. Various emission cross sections were measured in dependence of electron impact energy, and were made absolute by comparison with published data. In this way, absolute monochromator calibration could be obtained at several wave lengths. Photon emission in collisions of doubly charged Ne- and Ar ions with Ar, Kr or Xe at impact energies of 10-50 keV was investigated with the absolutely calibrated vuv spectrometer under polarisation-free arrangement. In the collision system (20Ne+2 + Xe) de-excitation after single electron capture was found to proceed with comparable probability via photon emission from excited NeII states and via transfer ionisation causing electron emission. State-resolved studies typical for single electron capture excitation on collisions of singly or multiply charged ions with hydrogen-like target atoms were carried out with the collision systems (20Ne+ + Li) and (20Ne+2 + Li) respectively. For both systems absolute emission cross sections for the strongest NeI and NeII lines emitted in the vuv range respectively were measured together with the respective total single electron capture cross sections. Absolute excitation cross sections were derived from the measured emission cross sections by correcting for the influence of the finite lifetime of the involved excited states. The obtained results could be explained satisfactorily with simple theoretical considerations on the basis of quasimolecular two state transitions. (author)

297

Quinone methide generation via photoinduced electron transfer.  

Science.gov (United States)

Photochemical activation of water-soluble 1,8-naphthalimide derivatives (NIs) as alkylating agents has been achieved by irradiation at 310 and 355 nm in aqueous acetonitrile. Reactivity in aqueous and neat acetonitrile has been extensively investigated by laser flash photolysis (LFP) at 355 nm, as well as by steady-state preparative irradiation at 310 nm in the presence of water, amines, thiols, and ethyl vinyl ether. Product distribution analysis revealed fairly efficient benzylation of the amines, hydration reaction, and 2-ethoxychromane generation, in the presence of ethyl vinyl ether, resulting from a [4 + 2] cycloaddition onto a transient quinone methide. Remarkably, we found that the reactivity was dramatically suppressed under the presence of oxygen and radical scavengers, such as thiols, which was usually associated with side product formation. In order to unravel the mechanism responsible for the photoreactivity of these NI-based molecules, a detailed LFP study has been carried out with the aim to characterize the transient species involved. LFP data suggest a photoinduced electron transfer (PET) involving the NI triplet excited state (?(max) 470 nm) of the NI core and the tethered quinone methide precursor (QMP) generating a radical ions pair NI(•-) (?(max) 410 nm) and QMP(•+). The latter underwent fast deprotonation to generate a detectable phenoxyl radical (?(max) 390 and 700 nm), which was efficiently reduced by the radical anion NI(•-), generating detectable QM. The mechanism proposed has been validated through a LFP investigation at 355 nm exploiting an intermolecular reaction between the photo-oxidant N-pentylnaphthalimide (NI-P) and a quaternary ammonium salt of a Mannich base as QMP (2a), in both neat and aqueous acetonitrile. Remarkably, these experiments revealed the generation of the model o-QM (?(max) 400 nm) as a long living transient mediated by the same reactivity pathway. Negligible QM generation has been observed under the very same conditions by irradiation of the QMP in the absence of the NI. Owing to the NIs redox and recognition properties, these results represent the first step toward new molecular devices capable of both biological target recognition and photoreleasing of QMs as alkylating species, under physiological conditions. PMID:21425810

Percivalle, Claudia; La Rosa, Andrea; Verga, Daniela; Doria, Filippo; Mella, Mariella; Palumbo, Manlio; Di Antonio, Marco; Freccero, Mauro

2011-05-01

298

Excited-state kinetics of the carotenoid S//1 state in LHC II and two-photon excitation spectra of lutein and beta-carotene in solution Efficient Car S//1 yields Chl electronic energy transfer via hot S//1 states?  

CERN Document Server

The excited-state dynamics of the carotenoids (Car) in light- harvesting complex II (LHC II) of Chlamydomonas reinhardtii were studied by transient absorption measurements. The decay of the Car S //1 population ranges from similar to 200 fs to over 7 ps, depending on the excitation and detection wavelengths. In contrast, a 200 fs Car S//1 yields Chlorophyll (Chl) energy transfer component was the dominant time constant for our earlier two-photon fluorescence up- conversion measurements (Walla, P.J. ; et al. J. Phys. Chem. B 2000, 104, 4799-4806). We also present the two-photon excitation (TPE) spectra of lutein and beta-carotene in solution and compare them with the TPE spectrum of LHC II. The TPE-spectrum of LHC II has an onset much further to the blue and a width that is narrower than expected from comparison to the S//1 fluorescence of lutein and beta-carotene in solution. Different environments may affect the shape of the S//1 spectrum significantly. To explain the blue shift of the TPE spectrum and the d...

Walla, P J; Linden, Patricia A; Ohta, Kaoru

2002-01-01

299

Minimal-excitation states for electron quantum optics using levitons.  

Science.gov (United States)

The on-demand generation of pure quantum excitations is important for the operation of quantum systems, but it is particularly difficult for a system of fermions. This is because any perturbation affects all states below the Fermi energy, resulting in a complex superposition of particle and hole excitations. However, it was predicted nearly 20 years ago that a Lorentzian time-dependent potential with quantized flux generates a minimal excitation with only one particle and no hole. Here we report that such quasiparticles (hereafter termed levitons) can be generated on demand in a conductor by applying voltage pulses to a contact. Partitioning the excitations with an electronic beam splitter generates a current noise that we use to measure their number. Minimal-excitation states are observed for Lorentzian pulses, whereas for other pulse shapes there are significant contributions from holes. Further identification of levitons is provided in the energy domain with shot-noise spectroscopy, and in the time domain with electronic Hong-Ou-Mandel noise correlations. The latter, obtained by colliding synchronized levitons on a beam splitter, exemplifies the potential use of levitons for quantum information: using linear electron quantum optics in ballistic conductors, it is possible to imagine flying-qubit operation in which the Fermi statistics are exploited to entangle synchronized electrons emitted by distinct sources. Compared with electron sources based on quantum dots, the generation of levitons does not require delicate nanolithography, considerably simplifying the circuitry for scalability. Levitons are not limited to carrying a single charge, and so in a broader context n-particle levitons could find application in the study of full electron counting statistics. But they can also carry a fraction of charge if they are implemented in Luttinger liquids or in fractional quantum Hall edge channels; this allows the study of Abelian and non-Abelian quasiparticles in the time domain. Finally, the generation technique could be applied to cold atomic gases, leading to the possibility of atomic levitons. PMID:24153178

Dubois, J; Jullien, T; Portier, F; Roche, P; Cavanna, A; Jin, Y; Wegscheider, W; Roulleau, P; Glattli, D C

2013-10-31

300

Direct conversion of graphite into diamond through electronic excited states  

International Nuclear Information System (INIS)

An ab initio total energy calculation has been performed for electronic excited states in diamond and rhombohedral graphite by the full-potential linearized augmented plane wave method within the framework of the local density approximation (LDA). First, calculations for the core-excited state in diamond have been performed to show that the ab initio calculations based on the LDA describe the wavefunctions in the electronic excited states as well as in the ground state quite well. Fairly good coincidence with both experimental data and theoretical prediction has been obtained for the lattice relaxation of the core exciton state. The results of the core exciton state are compared with nitrogen-doped diamond. Next, the structural stability of rhombohedral graphite has been investigated to examine the possibility of the transition into the diamond structure through electronic excited states. While maintaining the rhombohedral symmetry, rhombohedral graphite can be spontaneously transformed to cubic diamond. Total energy in the rhombohedral structure has been calculated as a function of cell volume V, c/a ratio and bond length between layers R. The adiabatic potential energy surfaces for the transition from rhombohedral graphite to diamond in the states after core excitation have been investigated. In core exciton state, the graphite structure is more stable than the diamond. In the valence hole state after the Auger decay process, in contrast, the graphite structure is r, in contrast, the graphite structure is remarkably unstable compared with the diamond. The conversion into diamond from graphite can be induced spontaneously even at room temperatures due to excited holes. The induced holes decrease the stable interlayer bond length, which can lower the activation energy for buckling displacement of the hexagonal bonds, and the activation energy becomes zero by increasing the concentration of holes up to 0.1/C atom. These results predict that diamond synthesis is possible by a core excitation through the Auger decay process. (topical review)

301

Dynamics of the excited state intramolecular charge transfer  

International Nuclear Information System (INIS)

The 6-dodecanoyl-2-dimethylaminonaphtalene (laurdan), a derivative of 6-propanoyl- 2-dimethylaminonaphthalene (prodan), has been used as a fluorescent probe in cell imaging, especially in visualizing the lipid rafts by the generalized polarization (GP) images, where GP=(I440-I490)/(I440+I490) with I being the fluorescence intensity. The fluorescence spectrum of laurdan is sensitive to its dipolar environment due to the intramolecular charge transfer (ICT) process in S1 state, which results in a dual emission from the locally excited (LE) and the ICT states. The ICT process and the solvation of the ICT state are very sensitive to the dipolar nature of the environment. In this work, the ICT of laurdan in ethanol has been studied by femtosecond time resolved fluorescence (TRF), especially TRF spectra measurement without the conventional spectral reconstruction method. TRF probes the excited states exclusively, a unique advantage over the pump/probe transient absorption technique, although time resolution of the TRF is generally lower than transient absorption and the TRF spectra measurement was possible only though the spectral reconstruction. Over the years, critical advances in TRF technique have been made in our group to achieve <50 fs time resolution with direct full spectra measurement capability. Detailed ICT and the subsequent solvation processes can be visualized unambiguously from the TRF spectra. Fig. 1 shobiguously from the TRF spectra. Fig. 1 shows the TRF spectra of laurdan in ethanol at several time delays. Surprisingly, two bands at 433 and 476 nm are clearly visible in the TRF spectra of laurdan even at T = 0 fs. As time increases, the band at 476 nm shifts to the red while its intensity increases. The band at 433 nm also shifts slightly to the red, but loses intensity as time increases. The intensity of the 476 nm band reaches maximum at around 5 ps, where it is roughly twice as intense as that at 0 fs, and stays constant until lifetime decay is noticeable. The spectra were fit by two log-normal functions, each corresponding to the fluorescence from the LE state and ICT state. Relative populations and the average frequency of the ICT state are shown in Fig.2 and Fig.3, which represents the ICT dynamics and the subsequent solvation process, respectively. The TRF spectra illustrate unambiguously how the ICT and the solvation processes take place for laurdan dissolved in ethanol. First of all, the ICT and the solvation occur in serial. Second, the ICT proceeds in several time scales exhibiting heterogeneity of the molecular conformation in liquid. About one third of the laurdan molecules undergoes ICT immediately in much less than <50 fs time scale. Rest of them undergoes ICT by two time constants, 6.4 ps and 28 ps. The ICT state then undergoes solvation process by 47 ps time constants. Because the molecule is large and flexible, fast inertial component was not observed. In conclusion, TRF spectra in femtosecond resolution reveal detailed intramolecular charge transfer process of laurdan. The ICT process shows a series of time scales due to the conformational heterogeneity in solution. (authors)

302

Electronically excited states of protonated aromatic hydrocarbons: phenanthrene and pyrene.  

Science.gov (United States)

The first and second electronic excited states (S1 and S2) of protonated phenanthrene and protonated pyrene, having the ??* nature, are strongly red-shifted compared to corresponding electronic transitions in neutral homologues. The CC2 calculations identify an out-of-plane deformation as the most important photochemical reaction coordinate in protonated phenanthrene as well as protonated benzene. It was shown that the excited S1 states of protonated phenanthrene and protonated benzene are unstable via a torsional motion, which provides a fast access to a S1-S0 conical intersection. From the conical intersection, a barrier-less reaction path directs the system back to the minimum of the S0 potential-energy surface. In contrast to the most stable isomer of protonated phenanthrene, the most stable structure of protonated pyrene shows planar structure in both the S1 and S2 excited states, without considerable geometry deformations. PMID:23441848

Saed, Behnaz; Omidyan, Reza

2013-03-28

303

Monte Carlo generators for excited electron and neutrino production  

International Nuclear Information System (INIS)

'EPEX' and 'NUPEX' are monte carlo event generators, written in standard FORTRAN 77, being able to simulate the production and the decay of the excited leptons e* and ?e* at the electron-proton-colliders HERA and LHC+LEP. A further option of the program 'EPEX' is to simulate the inelastic electron-proton-scattering via contact interactions as: e + p ? e + X. (orig./HSI)

304

Excitation of He(31D) by electron impact  

International Nuclear Information System (INIS)

We report new results of the Stokes parameters P1 to P4 and of the coherence parameters Plin, ?a, ?00 and Lperpendicularto for He(31D) excitation by electron impact in the energy range 40 eV to 81.6 eV obtained by the scattered electron-polarized photon coincidence technique. The present results are, where available, in fair agreement with previous meaasurements of other groups but disagree with the available calculations based on different models. (orig.)

305

Electron-impact excitation of gas-phase uracil  

Science.gov (United States)

The low lying excited states of uracil have been studied using electron energy-loss spectroscopy. In addition to the dipole allowed transitions to the singlet states, the two lowest triplet states are also observed. In the uracil molecule, the singlet electronic states have been found, being blue-shifted by about 0.5 eV as compared to the UV-absorption results.

Chernyshova, I. V.; Kontros, J. E.; Markush, P. P.; Borovik, A. A.; Shpenik, O. B.

2012-11-01

306

Rotational excitation of interstellar molecular ions by electrons  

OpenAIRE

Electrons are known to be efficient in rotationally exciting molecular ions in cold ionized media. Rotational effects have also been shown to affect the dissociative recombination (DR) process. Electron collisions are thus expected to play a significant role in the thermalization and dissociation dynamics of molecular ions, both in the laboratory and in space. Using the molecular R-matrix method combined with the Adiabatic-Nuclei-Rotation (ANR) approximation corrected for threshold and closed...

Faure, A.; Tennyson, J.; Kokoouline, V.; Greene, C. H.

2009-01-01

307

Electron impact excitation of Kr XXXII  

International Nuclear Information System (INIS)

Collision strengths (?) have been calculated for all 7750 transitions among the lowest 125 levels belonging to the 2s22p,2s2p2,2p3,2s23l,2s2p3l, and 2p23l configurations of boron-like krypton, Kr XXXII, for which the Dirac Atomic R-matrix Code has been adopted. All partial waves with angular momentum J?40 have been included, sufficient for the convergence of ? for forbidden transitions. For allowed transitions, a top-up has been included in order to obtain converged values of ? up to an energy of 500 Ryd. Resonances in the thresholds region have been resolved in a narrow energy mesh, and results for effective collision strengths (Y) have been obtained after averaging the values of ? over a Maxwellian distribution of electron velocities. Values of Y are reported over a wide temperature range below 107.3K, and the accuracy of the results is assessed. Values of Y are also listed in the temperature range 7.3?logTe(K)?9.0, obtained from the nonresonant collision strengths from the Flexible Atomic Code.

308

Magnetic excitations in an itinerant electron antiferromagnet Cr2As  

International Nuclear Information System (INIS)

Spin wave excitations in an itinerant electron antiferromagnet Cr2As was measured using neutron spectroscopy. Magnon dispersion is linear to the reciprocal lattice vector for small q, and the slope is 125 and 185 MeV A along c- and a-axis, respectively. The slope is about two times larger than the value predicted from the molecular field theory

309

Excitation of electron Langmuir frequency harmonics in the solar atmosphere  

Energy Technology Data Exchange (ETDEWEB)

An alternative mechanism for the excitation of electron Langmuir frequency harmonics as a result of the development of explosive instability in a weakly relativistic beam-plasma system in the solar atmosphere is proposed. The efficiency of the new mechanism as compared to the previously discussed ones is analyzed.

Fomichev, V. V.; Fainshtein, S. M.; Chernov, G. P. [Russian Academy of Sciences, Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (Russian Federation)

2013-05-15

310

Low energy excitations in the low-carrier heavy-electron system Yb4As3  

International Nuclear Information System (INIS)

Detailed inelastic neutron scattering studies on the low-carrier heavy-electron system Yb4As3 reveals that the magnetic response from this material for the energy transfer above about 0.3 meV is due to the spin excitations in the one-dimensional Yb3+ chains caused by the charge ordering. The energy scale of the heavy-electron anomaly in the transport property of Yb4As3 must be very small, if its response is significant compared to that of the spin excitations. Inelastic neutron scattering from a polycrystalline sample of Yb4 (As0.6P0.4)3 suggests that the similar phenomenon seems to occur in this semi-conducting mixed compound which shows similar heavy-electron-like behaviours of susceptibility and specific heat. (author)

311

Spin effects in intramolecular electron transfer in naproxen- N -methylpyrrolidine dyad  

Science.gov (United States)

The intramolecular electron transfer in the naproxen- N-methylpyrrolidine dyad has been investigated by spin chemistry methods. The existence of CIDNP in a high magnetic field points to electron transfer as a possible mechanism of the quenching of the excited state of a dyad. However, the failure to detect magnetic field effects on triplet yield makes us conclude that this quenching mechanism is not the only one. The observation of CIDNP effects in the dyad in the media of low polarity and the short risetime of triplet state formation indicate a potential role of exciplex in the quenching of the excited state of the dyad.

Magin, I. M.; Polyakov, N. E.; Khramtsova, E. A.; Kruppa, A. I.; Tsentalovich, Yu. P.; Leshina, T. V.; Miranda, M. A.; Nuin, E.; Marin, M. L.

2011-11-01

312

Solvent-controlled electron transfer in crystal violet lactone.  

Science.gov (United States)

Steady-state and picosecond time-resolved emission experiments are used to examine the excited-state charge transfer reaction of crystal violet lactone (CVL) in aprotic solvents. Solvatochromic analysis using a dielectric continuum model suggests dipole moments of 9-12 D for the initially excited (LE) state and ?24 D for the charge-transfer (CT) state. Intensities of steady-state emission as well as kinetic data provide free energies for the LE ? CT reaction that range from +12 kJ/mol in nonpolar solvents to -10 kJ/mol in highly polar solvents at 25 °C. Reaction rates constants, which lie in the range of 10-100 ns(-1) in most solvents, depend on both solvent polarity and solvent friction. In highly polar solvents, rates are correlated to solvation times in a manner that indicates that the reaction is a solvent-controlled electron transfer on an adiabatic potential surface having a modest barrier. PMID:20831148

Li, Xiang; Maroncelli, Mark

2011-04-28

313

Excitation transfer pathways in excitonic aggregates revealed by the stochastic Schr\\"odinger equation  

CERN Document Server

We derive the stochastic Schr\\"odinger equation for the system wave vector and use it to describe the excitation energy transfer dynamics in molecular aggregates. We suggest a quantum-measurement based method of estimating the excitation transfer time. Adequacy of the proposed approach is demonstrated by performing calculations on a model system. The theory is then applied to study the excitation transfer dynamics in a photosynthetic pigment-protein Fenna-Matthews-Olson (FMO) aggregate using both the Debye spectral density and the spectral density obtained from earlier molecular dynamics simulations containing strong vibrational high-frequency modes. The obtained results show that the excitation transfer times in the FMO system are affected by the presence of the vibrational modes, however the transfer pathways remain the same.

Abramavicius, Vytautas

2014-01-01

314

Excitation transfer pathways in excitonic aggregates revealed by the stochastic Schrödinger equation  

Energy Technology Data Exchange (ETDEWEB)

We derive the stochastic Schrödinger equation for the system wave vector and use it to describe the excitation energy transfer dynamics in molecular aggregates. We suggest a quantum-measurement based method of estimating the excitation transfer time. Adequacy of the proposed approach is demonstrated by performing calculations on a model system. The theory is then applied to study the excitation transfer dynamics in a photosynthetic pigment-protein Fenna-Matthews-Olson (FMO) aggregate using both the Debye spectral density and the spectral density obtained from earlier molecular dynamics simulations containing strong vibrational high-frequency modes. The obtained results show that the excitation transfer times in the FMO system are affected by the presence of the vibrational modes; however, the transfer pathways remain the same.

Abramavicius, Vytautas, E-mail: vytautas.ab@gmail.com; Abramavicius, Darius, E-mail: darius.abramavicius@ff.vu.lt [Faculty of Physics, Department of Theoretical Physics, Vilnius University, Saul?tekio 9, LT-10222 Vilnius (Lithuania)

2014-02-14

315

Photoinduced electron transfer between chlorophylls (a/b) and fullerenes (C60/C70) studied by laser flash photolysis.  

Science.gov (United States)

Photoinduced electron-transfer processes in the systems of chlorophylls (Chl) (chlorophyll-a [Chl-a] and chlorophyll-b) and fullerenes (C60/C70) in both polar and non-polar solvents have been investigated with nanosecond laser photolysis technique, observing the transient spectra in the visible/near-IR regions. By the excitation of Chl in benzonitrile (BN) it has been proved that electron transfer takes place from the triplet excited states of Chl to the ground states of C60/C70. By the excitation of C70 in BN electron transfer takes place from the ground states of Chl to the triplet excited state of C70. In both Chl the rate constants and quantum yields for the electron-transfer processes are as high as those of zinc porphyrins and zinc phthalocyanines, indicating that the long alkyl chains of Chl play no role in retarding the electron transfer. The rate constant for the electron-mediating process from the radical anion of C70 to octylviologen dication yielding the octylviologen radical cation was evaluated. The back electron-transfer process from the viologen radical cation to the radical cation of Chl-a takes place in a longer time-scale, indicating that a photosensitized electron-transfer/electron-mediating cycle is achieved. PMID:11460533

el-Khouly, M E; Araki, Y; Fujitsuka, M; Watanabe, A; Ito, O

2001-07-01

316

A questionable excited-state double-proton transfer mechanism for 3-hydroxyisoquinoline.  

Science.gov (United States)

Two excited state proton transfer mechanisms of 3-hydroxyisoquinoline (3HIQ) in cyclohexane and acetic acid (ACID) were investigated based on the time-dependent density functional theory (TDDFT), suggesting a different double-proton transfer mechanism from the one proposed previously (J. Phys. Chem. B, 1998, 102, 1053). Instead of the formation of keto-enol complexes for 3HIQ self-association in cyclohexane, our theoretical results predicted that 3HIQ self-association exists in two forms: the normal form (enol/enol) and the tautomer form (keto/keto) in cyclohexane. A high barrier (37.023 kcal mol(-1)) between the 3HIQ enol monomer and 3HIQ keto monomer form indicated that the 3HIQ keto monomer in the ground state should not exist. In addition, the constructed potential energy surfaces of the ground state and excited state have been used to explain the proton transfer process. Upon optical excitation, the enol/enol form is excited to the first excited state, then transfers one proton, in turn, transition to the ground state to transfer another proton. A relatively low barrier (8.98 kcal mol(-1)) demonstrates two stable structures in the ground state. In view of the acetic acid solvent effect, two protons of 3HIQ/ACID transfer along the dihydrogen bonds in the first excited state, which is a different transfer mechanism to 3HIQ self-association. In addition, the proton transfer process provides a possible explanation for the fluorescence quenching observed. PMID:25418334

Zhao, Jinfeng; Chen, Junsheng; Cui, Yanling; Wang, Jing; Xia, Lixin; Dai, Yumei; Song, Peng; Ma, Fengcai

2015-01-14

317

Water-catalyzed excited-state proton-transfer reactions in 7-azaindole and its analogues.  

Science.gov (United States)

The mechanism of the water-catalyzed excited-state proton-transfer (ESPT) reaction for 7-azaindole (7AI) has long been investigated, but there are some controversial viewpoints. Recently, owing to the superiority of sensing biowaters in proteins by a 7AI analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed ESPT in 7AI and its analogues in an attempt to unify the mechanism. Herein, a series of 7AI analogues and their methylated derivatives were synthesized to carry out a systematic study on pKa, pKa*, and the associated fluorescence spectroscopy and dynamics. The results conclude that all 7AI derivatives undergo water-catalyzed ESPT in neutral water. However, for those derivatives with -H (7AI) and a electron-donating substituent at C(3), they follow water-catalyzed ESPT to form an excited N(7)-H proton-transfer tautomer, T*. T* is rapidly protonated to generate an excited cationic (TC*) species. TC* then undergoes a fast deactivation to the N(1)-H normal species in the ground state. Conversely, protonation in T* is prohibited for those derivatives with an electron-withdrawing group at the C(2) or C(3) or with the C(2) atom replaced by an electron-withdrawing nitrogen atom (N(2) in, e.g., 2,7-diazatryptophan), giving a prominent green T* emission. Additional support is given by the synthesis of the corresponding N(7)-CH3 tautomer species, for which pKa* of the cationic form, that is, the N(7)-CH3N(1)-H(+) species, is measured to be much greater than 7.0 for those with electron-donating C(3) substituents, whereas it is lower than 7.0 upon anchoring electron-withdrawing groups. For 7AI, the previously missing T* emission is clearly resolved with a peak wavelength at 530 nm in the pH interval of 13.0-14.3 (H- 14.2). PMID:25225896

Wu, Yu-Sin; Huang, Huai-Ching; Shen, Jiun-Yi; Tseng, Huan-Wei; Ho, Jr-Wei; Chen, You-Hua; Chou, Pi-Tai

2015-02-12

318

A strap strategy for construction of an excited-state intramolecular proton transfer (ESIPT) system with dual fluorescence.  

Science.gov (United States)

An amine-embedded flexible alkyl strap has been incorporated into an emissive boryl-substituted dithienylpyrrole skeleton as a new entity of excited-state intramolecular proton transfer (ESIPT) chromophores. The ?-electron system shows a dual emission, which covers a wide range of the visible region depending on the solvent polarity. The incorporation of the aminoalkyl strap as well as the terminal boryl groups efficiently stabilize the zwitterionic excited-state species resulting from the ESIPT even in an aqueous medium. PMID:24931485

Suzuki, Naoya; Fukazawa, Aiko; Nagura, Kazuhiko; Saito, Shohei; Kitoh-Nishioka, Hirotaka; Yokogawa, Daisuke; Irle, Stephan; Yamaguchi, Shigehiro

2014-07-28

319

Energy transfer from the host excitations to Ce{sup 3+} ions in scandium borate  

Energy Technology Data Exchange (ETDEWEB)

Efficient energy transfer from the host excitations (self-trapped excitons, STE) excited at {lambda}{approx}160 nm to Ce{sup 3+} impurity ions was observed in scandium borate (ScBO{sub 3}). The luminescence and excitation spectra as well as time-resolved emission data were obtained. Energy transfer rates from the STE to Ce{sup 3+} of about 10{sup 8} s{sup -1} are observed and these are within an order of magnitude of estimates based on dipole-dipole energy transfer.

Feofilov, S.P. [Ioffe Physical-Technical Institute, St. Petersburg (Russian Federation)]. E-mail: Sergey.Feofilov@pop.ioffe.rssi.ru; Zhou, Y. [Department of Physics and Astronomy, University of Georgia, Athens, GA 30602 (United States); Jeong, J.Y. [Department of Chemistry, Oregon State University, Corvallis, OR 97331 (United States); Keszler, D.A. [Department of Chemistry, Oregon State University, Corvallis, OR 97331 (United States); Meltzer, R.S. [Department of Physics and Astronomy, University of Georgia, Athens, GA 30602 (United States)

2007-07-15

320

Dissociative electron attachment to laser-excited benzene  

International Nuclear Information System (INIS)

We have conducted comprehensive measurements on enhanced electron attachment to ArF and KrF laser-excited benzene in the presence of Ar and N2 buffer gases. At both these laser lines, two-photon absorption leads to excitation of benzene to energies above its ionization potential. Such excitations have been shown to lead to a population of long-lived, core-excited high-Rydberg states in addition to the ionization of the molecule. Present measurements on the dependence of negative ion yield on laser fluence, benzene pressure, and applied electric field verify that the observed negative ion formation is due to the attachment of the photoelectrons to the concomitantly produced high-Rydberg states. Using a rate equation analysis, the electron attachment rate constant for the core-excited Rydberg states was estimated to be of the order of 10-4-10-3 cm3 s-1. Laser photoionization cross sections were also estimated, and the cross section at the KrF laser line is in agreement (author)

321

Dynamics of two-electron excitations in helium  

Energy Technology Data Exchange (ETDEWEB)

Excitation of both electrons in helium offers a unique window for studying electron correlation at the most basic level in an atom in which these two electrons and the nucleus form a three-body system. The authors utilized the first light available at the U-8 undulator-SGM monochromator beamline to investigate the dynamic parameters, partial cross sections, differential cross sections, and photoelectron angular distribution parameters ({beta}), with a high resolving power for the photon beam and at the highly differential level afforded by the use of their electron spectrometer. In parallel, they carried out detailed calculations of the relevant properties by a theoretical approach that is based on the hyperspherical close-coupling method. Partial photoionization cross sections {sigma}{sub n}, and photoelectron angular distributions {beta}{sub n} were measured for all possible final ionic states He{sup +}(n) in the region of the double excitations N(K,T){sup A} up to the N=5 threshold. At a photon energy bandpass of 12 meV below the thresholds N=3, 4, and 5, this level of differentiation offers the most critical assessment of the dynamics of the two-electron excitations to date. The experimental data were seen to be very well described by the most advanced theoretical calculations.

Caldwell, C.D.; Menzel, A.; Frigo, S.P. [Univ. of Central Florida, Orlando, FL (United States)] [and others

1997-04-01

322

Failures of TDDFT in describing the lowest intramolecular charge-transfer excitation in para-nitroaniline  

DEFF Research Database (Denmark)

We investigate the failure of Time{Dependent Density Functional Theory (TDDFT) with the CAM{B3LYP exchange{correlation (xc) functional coupled to the Polarizable Embedding (PE) scheme (PE-CAM-B3LYP) in reproducing the solvatochromic shift of the lowest intense charge{transfer excitation in para{nitroaniline (pNA) in water by comparing with results obtained with the Coupled Cluster Singles and Doubles (CCSD) model also coupled to the Polarizable Embedding scheme (PE-CCSD). We determine the amount of charge separation in the ground and excited charge{transfer state with both methods by calculating the electric dipole moments in the gas phase and for 100 solvent congurations. We find that CAM-B3LYP overestimates the amount of charge separation inherent in the ground state and TDDFT/CAM-B3LYP drastically underestimates this amount in the excited charge-transfer state. As the errors in the solvatochromatic shift are found to be inverse proportional to thechange in dipole moment upon excitation, we conclude that the flaws in the description of the solvatochromic shift of this excitation are related to TDDFT itself and how it responds to the solvent e¿ects modelled by the PE scheme. We recommend therefore to benchmark results of TDDFT calculations with CAM-B3LYP for intramolecular charge{transfer excitations in molecular systems similar to pNA against higher{level ab initio wave function methods, like, e.g., CCSD, prior to their use. Using the calculated change in dipole moment upon excitation as a measure for charge{transfer character, we furthermore conrm that the di¿erence between excitation energies calculated with TDDFT and with the Tamm-Danco¿ approximation (TDA) to TDDFT is indeed correlated with the charge-transfer character of a given electronic transition both in vacuo and in solution. This is supported by a corresponding correlation between the change in dipole moment and the size of the index diagnostic for the investigated CT excitation.

Eriksen, Janus J.; Sauer, Stephan P. A.

2013-01-01

323

Reaction coordinates for electron transfer reactions  

International Nuclear Information System (INIS)

The polarization fluctuation and energy gap formulations of the reaction coordinate for outer sphere electron transfer are linearly related to the constant energy constraint Lagrangian multiplier m in Marcus' theory of electron transfer. The quadratic dependence of the free energies of the reactant and product intermediates on m and m+1, respectively, leads to similar dependence of the free energies on the reaction coordinates and to the same dependence of the activation energy on the reorganization energy and the standard reaction free energy. Within the approximations of a continuum model of the solvent and linear response of the longitudinal polarization to the electric field in Marcus' theory, both formulations of the reaction coordinate are expected to lead to the same results.

324

Electron dynamics and energy dissipation in highly excited dielectrics  

Science.gov (United States)

When dielectrics are irradiated with an ultrashort laser pulse or a swift heavy ion, the transient density of electrons in the conduction band increases considerably. This density is a crucial parameter for the subsequent behavior of the material: After ion irradiation, it influences energy dissipation to the lattice as well as the energy transport to the outer track. For the case of laser irradiation on a timescale of about hundred femtoseconds, the free-electron density increase due to irradiation also determines the further absorption of the pulse energy. Additionally, the distribution function of the excited electrons may influence energy absorption and dissipation.

Rethfeld, B.; Rämer, A.; Brouwer, N.; Medvedev, N.; Osmani, O.

2014-05-01

325

Electron excitations and sum rules for multipole transitions  

International Nuclear Information System (INIS)

Form factors and charge densities of the hydrodynamical model of the nucleus used in electron excitation are obtained from the multipole sum rules by assuming the partial exhaustion of the sum rules by one single state. Corrections are indicated. Simple relations are obtained for the transition radii Rsub(tr)(E1), resulting in a model-independent ratio of Rsub(tr)(E2)/Rsub(tr)(E0)=0.84. The static quadrupole moments of the giant resonances are shown to increase with multipolarity and to decrease with excitation energy

326

Correlations in highly excited two-electron atoms - planetary behavior  

International Nuclear Information System (INIS)

The onset of strong mixing of atomic configurations in core/excited autoionization states of Ba has been observed using multiphoton isolated core excitation (ICE). The behavior of the different atomic configurations is categorized into two groups: states where n is much greater than m; and states where n is approximately equal to m. The spectral characteristics of 6s15 to 10sn-prime s transitions is in the case of n is much greater than m described within the framework of the independent electron model of Percival (1977). Estimates of the relative autoionization rates corresponding to the different atomic configurations are given in an appendix. 10 references

327

Promoting Interspecies Electron Transfer with Biochar  

OpenAIRE

Biochar, a charcoal-like product of the incomplete combustion of organic materials, is an increasingly popular soil amendment designed to improve soil fertility. We investigated the possibility that biochar could promote direct interspecies electron transfer (DIET) in a manner similar to that previously reported for granular activated carbon (GAC). Although the biochars investigated were 1000 times less conductive than GAC, they stimulated DIET in co-cultures of Geobacter metallireducens with...

Chen, Shanshan; Rotaru, Amelia-elena; Shrestha, Pravin Malla; Malvankar, Nikhil S.; Liu, Fanghua; Fan, Wei; Nevin, Kelly P.; Lovley, Derek R.

2014-01-01

328

Electron transfer theory revisit: Quantum solvation effect  

CERN Document Server

The effect of solvation on the electron transfer (ET) rate processes is investigated on the basis of the exact theory constructed in J. Phys. Chem. B Vol. 110, (2006); quant-ph/0604071. The nature of solvation is studied in a close relation with the mechanism of ET processes. The resulting Kramers' turnover and Marcus' inversion characteristics are analyzed accordingly. The classical picture of solvation is found to be invalid when the solvent longitudinal relaxation time is short compared with the inverse temperature.

Han, P; Cui, P; Mo, Y; He, G; Yan, Y J; Han, Ping; Xu, Rui-Xue; Cui, Ping; Mo, Yan; He, Guozhong; Yan, YiJing

2006-01-01

329

Quantum coherent contributions in biological electron transfer  

OpenAIRE

Many biological electron transfer (ET) reactions are mediated by metal centres in proteins. NADH:ubiquinone oxidoreductase (complex I) contains an intramolecular chain of seven iron-sulphur (FeS) clusters, one of the longest chains of metal centres in biology and a test case for physical models of intramolecular ET. In biology, intramolecular ET is commonly described as a diffusive hopping process, according to the semi-classical theories of Marcus and Hopfield. However, rec...

Dorner, Ross; Goold, John; Heaney, Libby; Farrow, Tristan; Roberts, Philippa G.; Hirst, Judy; Vedral, Vlatko

2011-01-01

330

Electronic excitation of ground state atoms by collision with heavy gas particles  

Science.gov (United States)

Most of the important chemical reactions which occur in the very high temperature air produced around space vehicles as they enter the atmosphere were investigated both experimentally and theoretically, to some extent at least. One remaining reaction about which little is known, and which could be quite important at the extremely high temperatures that will be produced by the class of space vehicles now contemplated - such as the AOTV - is the excitation of bound electron states due to collisions between heavy gas particles. Rates of electronic excitation due to free electron collisions are known to be very rapid, but because these collisions quickly equilibrate the free and bound electron energy, the approach to full equilibrium with the heavy particle kinetic energy will depend primarily on the much slower process of bound electron excitation in heavy particle collisions and the subsequent rapid transfer to free electron energy. This may be the dominant mechanism leading to full equilibrium in the gas once the dissociation process has depleted the molecular states so the transfer between molecular vibrational energy and free electron energy is no longer available as a channel for equilibration of free electron and heavy particle kinetic energies. Two mechanisms seem probable in electronic excitation by heavy particle impact. One of these is the collision excitation and deexcitation of higher electronic states which are Rydberg like. A report, entitled 'Semi-Classical Theory of Electronic Excitation Rates', was submitted previously. This presented analytic expressions for the transition probabilities, assuming that the interaction potential is an exponential repulsion with a perturbation ripple due to the dipole-induced dipole effect in the case of neutral-neutral collisions, and to the ion-dipole interaction in the case of ion-neutral collisions. However the above may be, there is little doubt that excitation of ground state species by collision occurs at the point where the initial and final potentials cross, or at least come very close. Therefore, this mechanism would be applicable to the case where a gas is initially at very low temperature suddenly subjected to high energy heavy particle bombardment. This situation would model the measurement of excitation cross section by molecular beam techniques, for example. The purpose is to report values of cross sections and rate coefficients for collision excitation of ground state atoms estimated with the Landau-Zener transition theory and to compare results with measurement of excitation cross sections for a beam of Hydrogen atoms impacting Argon atom targets. Some very dubious approximations are used, and the comparison with measurement is found less than ideal, but results are at least consistent within order of magnitude. The same model is then applied to the case of N-N atom collisions, even though the approximations then become even more doubtful. Still the rate coefficients obtained are at least plausible in both magnitude and functional form, and as far as I am aware these are the only estimates available for such rate coefficients.

Hansen, C. Frederick

1993-01-01

331

Electronic excitation of the lB2 state of furan by electron impact  

International Nuclear Information System (INIS)

We report on recent results obtained in studies involving electronically inelastic electron scattering from furan. In particular, we considered the electronic transition from ground state to the 1B2 excited state. The scattering calculations employed the Schwinger multichannel method implemented with pseudopotentials and were carried out up to a nine-state close-coupling plus polarization level of approximation.

332

Study of excitation transfer in fluid molecular media: 2. Experimental aspects  

International Nuclear Information System (INIS)

The validity of our kinetic theory of excitation transfer between moving donors and acceptors is examined from donor fluorescence decay measurements performed in solutions with various acceptors. The experimental existence of specific fast and slow diffusion regimes has been made conspicuous. Experimentally deduced transfer lengths are discussed as a function of the viscosity of the solvent and of the reactive interaction responsible for excitation transfer. The characteristic parameters of the naphthalene-anthracene exchange interaction-cooperating in conjunction with a long-range dipole-dipole mechanism - could be deduced from transfer rate measurements in solvents of different viscosities

333

Electron impact ionization and excitation of uracil molecules  

International Nuclear Information System (INIS)

The cross-sections of the formation of positive and negative ions of uracil, a nitrogenous base of nucleic acids, are obtained experimentally. The values for negative ions were shown to reach their maximum of 5.0x10-18 cm2 at an energy of bombarding electrons of 1.1 eV. The magnitudes and the energy dependence were determined for the cross-section of formation of positive uracil ions in the electron energy interval from the formation threshold to 200 eV. The ionization cross-section peak of (1.0±0:1)x10-15 cm2 was found at an energy of 95 eV. The luminescence spectrum for isolated uracil molecules consisting of about 20 spectral bands and lines emitted under the action of slow electrons was obtained in the wavelength range 200-500 nm. The uracil radiation spectrum was shown to be driven by the processes of molecular dissociative excitation, dissociative excitation with ionization, and excitation of electron levels in the initial molecule and the molecular ion. The biophysical consequences of the results obtained are discussed.

334

Excited electronic states and photophysics of uracil-water complexes  

International Nuclear Information System (INIS)

The electronically excited singlet states of complexes of uracil with one water molecule have been studied theoretically using ab initio multireference configuration interaction methods. In agreement with previous theoretical and experimental results, four cyclic isomers of uracil forming hydrogen bonds with the water molecule have been located with energies within 0.2 eV from the lowest energy isomer. Focus has been given on the mechanism for radiationless decay to the ground state after initial UV absorption and on the effect of complexation with water on previously reported radiationless decay pathways. Features on the excited state potential energy surfaces, such as minima, transition states and conical intersections, have been located for all isomers and compared with those of free uracil. The hydrogen-bonded water molecule changes the relative energies of these features and may lead to different excited state dynamics and lifetimes, in agreement with experimental observations

335

Electron transfer data for Cq+ and Oq+ ions in collisions with H, H2 and He targets  

International Nuclear Information System (INIS)

The present status is reviewed of investigations on total and partial (nl) cross sections for electron transfer processes of Cq+ and Oq+ ions, which are the most dominant impurity ions in plasma apparatuses, colliding with H, H2 and He atoms, including some new additional cross sections calculated for these ions. Urgent needs for new data related to diagnostics and modelling of high temperature plasmas are pointed out: 1. electron transfer into higher (nl) states, 2. electron transfer involving excited target species, in particular excited atomic hydrogen, 3. collision between impurity ions and protons or alpha particles. (orig.)

336

Coincidence study of excitation of cadmium atoms by electron impact  

International Nuclear Information System (INIS)

We present experimental values of the electron impact coherence parameters (EICP) and reduced Stokes parameters for excitation of 51P1 state of cadmium atoms. The results have been obtained using electron-photon coincidence technique for incident electron energies 80 eV and 60 eV and electron scattering angles in the range of 5 deg. to 50 deg. We also present an additional set of data for electron energy 100 eV and scattering angle 50 deg. which complements our previous results. All the experimental values are compared with theoretical relativistic distorted-wave approximation (RDWA) calculations. The first Born approximation (FBA) predictions of the alignment angle are also presented. The theoretical results are in good qualitative agreement with the experimental data

337

Electron transfer and reaction mechanism of laccases.  

Science.gov (United States)

Laccases are part of the family of multicopper oxidases (MCOs), which couple the oxidation of substrates to the four electron reduction of O2 to H2O. MCOs contain a minimum of four Cu's divided into Type 1 (T1), Type 2 (T2), and binuclear Type 3 (T3) Cu sites that are distinguished based on unique spectroscopic features. Substrate oxidation occurs near the T1, and electrons are transferred approximately 13 Å through the protein via the Cys-His pathway to the T2/T3 trinuclear copper cluster (TNC), where dioxygen reduction occurs. This review outlines the electron transfer (ET) process in laccases, and the mechanism of O2 reduction as elucidated through spectroscopic, kinetic, and computational data. Marcus theory is used to describe the relevant factors which impact ET rates including the driving force, reorganization energy, and electronic coupling matrix element. Then, the mechanism of O2 reaction is detailed with particular focus on the intermediates formed during the two 2e(-) reduction steps. The first 2e(-) step forms the peroxide intermediate, followed by the second 2e(-) step to form the native intermediate, which has been shown to be the catalytically relevant fully oxidized form of the enzyme. PMID:25572295

Jones, Stephen M; Solomon, Edward I

2015-03-01

338

77 FR 40459 - Electronic Fund Transfers (Regulation E); Correction  

Science.gov (United States)

...Documents. Prices of new books are listed in the first FEDERAL...CFPB-2011-0009] RIN 3170-AA15 Electronic Fund Transfers (Regulation...6194), which implements the Electronic Fund Transfer Act, and the...in the interim final rule, Electronic Fund Transfers...

2012-07-10

339

Imaging Excited State Dynamics with 2d Electronic Spectroscopy  

Science.gov (United States)

Excited states in the condensed phase have extremely high chemical potentials making them highly reactive and difficult to control. Yet in biology, excited state dynamics operate with exquisite precision driving solar light harvesting in photosynthetic complexes though excitonic transport and photochemistry through non-radiative relaxation to photochemical products. Optimized by evolution, these biological systems display manifestly quantum mechanical behaviors including coherent energy transfer, steering wavepacket trajectories through conical intersections and protection of long-lived quantum coherence. To image the underlying excited state dynamics, we have developed a new spectroscopic method allowing us to capture excitonic structure in real time. Through this method and other ultrafast multidimensional spectroscopies, we have captured coherent dynamics within photosynthetic antenna complexes. The data not only reveal how biological systems operate, but these same spectral signatures can be exploited to create new spectroscopic tools to elucidate the underlying Hamiltonian. New data on the role of the protein in photosynthetic systems indicates that the chromophores mix strongly with some bath modes within the system. The implications of this mixing for excitonic transport will be discussed along with prospects for transferring underlying design principles to synthetic systems.

Engel, Gregory S.

2012-06-01

340

Theoretical studies on the reaction pathways of electronically excited DAAF  

Energy Technology Data Exchange (ETDEWEB)

The use of temporally and spectrally shaped ultrafast laser pulses to initiate, as well as detect, high explosives is being explored at Los Alamos. High level ab initio calculations, presented here, are employed to help guide and interpret the experiments. The ground and first excited electronic states of 3,3{prime}-diamino-4,4{prime}-azoxyfurazan (DAAF) are investigated using complete active space self-consistent field (CASSCF) and time-dependent density functional theory (TD-DFT). The geometrical and energetic character of the excited state minima, conical intersections and reaction pathways of DAAF are described. Two radiative and two non-radiative excited state population quenching mechanisms are outlined, and possible pathways for photochemical and spectroscopic control are discussed. The use of laser light to control chemical reactions has many applications. The initiation and the detection of explosives are two such applications currently under development at Los Alamos. Though inherently experimental, the project can be aided by theory through both prediction and interpretation. When the laser light is in the UV/visible region of the electromagnetic spectrum, the absorbing molecule is excited electronically and excitation decay may occur either radiatively (fluorescence or phosphorescence) or non-radiatively (through internal conversion). In many cases decay of the excitation occurs through a mixture of processes, and maximizing the desired result requires sophisticated laser pulses whose amplitude has been optimally modulated in time and/or frequency space. Control of cis-stilbene photochemistry was recently demonstrated in our group, and we aim to extend this work to high explosive compounds. Maximizing radiative decay leads to increased fluorescence quantum yields and enhances the possibility of spectral detection of the absorbing molecule. Maximizing non-radiative decay can lead to chemistry, heating of the sample and possibly detonation initiation in an explosive compound. Here we describe high level quantum chemistry calculations aimed at mapping the electronic states involved in excitation of 3,3{prime}-Diamino-4,4{prime}-azoxyfurazan (DAAF) with 400-nm light. DAAF is a high-nitrogen high explosive that is of interest for its relative insensitivity to shock compression. The goal of the theoretical work described here is to determine the competing pathways for radiative and non-radiative electronic state quenching in an effort to help guide spectroscopic experiments being conducted in tandem.

Quenneville, Jason M [Los Alamos National Laboratory; Moore, David S [Los Alamos National Laboratory

2009-01-01

341

Electron attachment to excited states of silane: Implications for plasma processing discharges  

International Nuclear Information System (INIS)

Observation of enhanced negative-ion formation in ArF endash excimer endash laser irradiated silane was reported in a recent paper [L. A. Pinnaduwage, M. Z. Martin, and L. G. Christophorou, Appl. Phys. Lett. 65, 2571 (1994)]. In that paper, preliminary evidence was presented to show that highly excited electronic states of silane or its photofragments could be responsible for the observed enhanced negative-ion formation. In the present paper, we report evidence, obtained using a new experimental technique, that the electron attaching species are high-Rydberg (HR) states of silane indirectly populated via laser irradiation and show that an absolute lower bound for the corresponding electron attachment rate constant is ?4x10-7cm3s-1. The initial capture of the electron by the HR states is likely to be a diabatic process and the large polarizabilities associated with the HR states appear to be responsible for the observed large electron attachment rate constants. We also measured electron attachment to thermally excited vibrational states of the ground electronic state of silane, which showed no measurable electron attachment up to 750 K. Implications of these observations in modeling of silane discharges used for plasma processing of amorphous silicon are discussed. It is also pointed out that large negative ion formation observed in many open-quotes weakly electronegativeclose quotes plasma processing gas discharges could be due to enessing gas discharges could be due to enhanced electron attachment to HR states: such states could be populated via direct electron impact and/or via excitation transfer from the metastable states of rare gases that are commonly used in these processing discharges. copyright 1997 American Institute of Physics

342

Photonically excited electron emission from modified graphitic nanopetal arrays  

International Nuclear Information System (INIS)

Efficient electron emission for energy conversion requires a low work function and a stable emitter material. The work function of graphene-based carbon materials can decrease significantly by intercalation with alkali metals, thus increasing their emission current. In this work, electron emission from potassium-intercalated carbon nanosheet extensions grown on electrode graphite is investigated. These petal-like structures, composed of 5–25 layers of graphene, are synthesized using microwave plasma chemical vapor deposition. Samples are intercalated with potassium, and a hemispherical energy analyzer is used to measure the emission intensity caused by both thermal and photonic excitation. The emission from the potassium-intercalated structures is found to consistently decrease the work function by 2.4 to 2.8 eV relative to non-intercalated samples. High emission intensity induced by photonic excitation from a solar simulator, with a narrow electron energy distribution relative to established theory, suggests that electron scattering decreases emitted electron energy as compared to surface photoemission. A modified photoemission theory is applied to account for electron scattering, and the sample work function and mean number of scattering events are used as parameters to fit theory to experimental data. The thermal stability of the intercalated nanopetals is investigated, and after an initial heating and cooling cycle, the samples are stable at low temperatures.mples are stable at low temperatures.

343

Multiplication Processes of Electronic Excitations in PbCl2 Crystals Excited by Vacuum Ultraviolet Radiation  

Science.gov (United States)

The excitation spectra for two intrinsic luminescence bands at 3.76 eV (UV) and 2.62 eV (BG) in PbCl2 have been measured using synchrotron radiation in the energy range up to 25 eV at 10 K. Since the UV and BG bands are connected to excitonic luminescence and electron-hole (e-h) recombination luminescence, respectively, an analysis of the difference in their excitation spectra allows us to separate excitonic and e-h mechanisms in the multiplication of electronic excitations. It is found that the intensity of the UV emission is sharply increased at 9.8 eV, which is higher than twice the band gap energy. Secondary excitons are formed by inelastic scattering of hot photoelectrons in the region above 9.8 eV. On the other hand, the BG emission is gradually enhanced in the range of 9-12.5 eV, and the intensity doubles at around 12.5 eV. This result indicates that the creation of two e-h pairs per photon predominantly causes in the region above 12.5 eV. The e-h multiplication originates not only in inelastic scattering of hot photoelectrons in the 12.5-25 eV range but also in interband Auger recombination of holes in the Cl- core levels with valence electrons in the 18-25 eV range.

Kitaura, Mamoru; Itoh, Minoru; Nakagawa, Hideyuki; Fujita, Masami

2003-03-01

344

Promoting Interspecies Electron Transfer with Biochar  

Science.gov (United States)

Biochar, a charcoal-like product of the incomplete combustion of organic materials, is an increasingly popular soil amendment designed to improve soil fertility. We investigated the possibility that biochar could promote direct interspecies electron transfer (DIET) in a manner similar to that previously reported for granular activated carbon (GAC). Although the biochars investigated were 1000 times less conductive than GAC, they stimulated DIET in co-cultures of Geobacter metallireducens with Geobacter sulfurreducens or Methanosarcina barkeri in which ethanol was the electron donor. Cells were attached to the biochar, yet not in close contact, suggesting that electrons were likely conducted through the biochar, rather than biological electrical connections. The finding that biochar can stimulate DIET may be an important consideration when amending soils with biochar and can help explain why biochar may enhance methane production from organic wastes under anaerobic conditions. PMID:24846283

Chen, Shanshan; Rotaru, Amelia-Elena; Shrestha, Pravin Malla; Malvankar, Nikhil S.; Liu, Fanghua; Fan, Wei; Nevin, Kelly P.; Lovley, Derek R.

2014-01-01

345

Promoting interspecies electron transfer with biochar  

DEFF Research Database (Denmark)

Biochar, a charcoal-like product of the incomplete combustion of organic materials, is an increasingly popular soil amendment designed to improve soil fertility. We investigated the possibility that biochar could promote direct interspecies electron transfer (DIET) in a manner similar to that previously reported for granular activated carbon (GAC). Although the biochars investigated were 1000 times less conductive than GAC, they stimulated DIET in co-cultures of Geobacter metallireducens with Geobacter sulfurreducens or Methanosarcina barkeri in which ethanol was the electron donor. Cells were attached to the biochar, yet not in close contact, suggesting that electrons were likely conducted through the biochar, rather than biological electrical connections. The finding that biochar can stimulate DIET may be an important consideration when amending soils with biochar and can help explain why biochar may enhance methane production from organic wastes under anaerobic conditions.

Chen, Shanshan; Rotaru, Amelia-Elena

2014-01-01

346

Electron impact excitation of one- and two-electron positive ions  

International Nuclear Information System (INIS)

The distorted-wave polarised-orbital (DWPO) model is applied to the evaluation of electron impact excitation cross sections for the lowest s ? s and s ? p transitions in one-electron positive ions (2 12+, Ca18+ and Fe24+). The results for He+(2p) are compared with experiment, and the other results with available theoretical models, from excitation threshold to high energies. (author)

347

Extremely confined gap surface-plasmon modes excited by electrons.  

DEFF Research Database (Denmark)

High-spatial and energy resolution electron energy-loss spectroscopy (EELS) can be used for detailed characterization of localized and propagating surface-plasmon excitations in metal nanostructures, giving insight into fundamental physical phenomena and various plasmonic effects. Here, applying EELS to ultra-sharp convex grooves in gold, we directly probe extremely confined gap surface-plasmon (GSP) modes excited by swift electrons in nanometre-wide gaps. We reveal the resonance behaviour associated with the excitation of the antisymmetric GSP mode for extremely small gap widths, down to ~5 nm. We argue that excitation of this mode, featuring very strong absorption, has a crucial role in experimental realizations of non-resonant light absorption by ultra-sharp convex grooves with fabrication-induced asymmetry. The occurrence of the antisymmetric GSP mode along with the fundamental GSP mode exploited in plasmonic waveguides with extreme light confinement is a very important factor that should be taken into account in the design of nanoplasmonic circuits and devices.

Raza, SØren; Stenger, Nicolas

2014-01-01

348

Energy shift of collective electron excitations in highly corrugated graphitic nanostructures: Experimental and theoretical investigation  

International Nuclear Information System (INIS)

Effect of corrugation of hexagonal carbon network on the collective electron excitations has been studied using optical absorption and X-ray photoelectron spectroscopy in conjunction with density functional theory calculations. Onion-like carbon (OLC) was taken as a material, where graphitic mantle enveloping agglomerates of multi-shell fullerenes is strongly curved. Experiments showed that positions of ? and ??+?? plasmon modes as well as ? ? ?* absorption peak are substantially redshifted for OLC as compared with those of highly ordered pyrolytic graphite and thermally exfoliated graphite consisted of planar sheets. This effect was reproduced in behavior of dielectric functions of rippled graphite models calculated within the random phase approximation. We conclude that the energy of electron excitations in graphitic materials could be precisely tuned by a simple bending of hexagonal network without change of topology. Moreover, our investigation suggests that in such materials optical exciton can transfer energy to plasmon non-radiatively

349

Energy shift of collective electron excitations in highly corrugated graphitic nanostructures: Experimental and theoretical investigation  

Energy Technology Data Exchange (ETDEWEB)

Effect of corrugation of hexagonal carbon network on the collective electron excitations has been studied using optical absorption and X-ray photoelectron spectroscopy in conjunction with density functional theory calculations. Onion-like carbon (OLC) was taken as a material, where graphitic mantle enveloping agglomerates of multi-shell fullerenes is strongly curved. Experiments showed that positions of ? and ??+?? plasmon modes as well as ? ? ?* absorption peak are substantially redshifted for OLC as compared with those of highly ordered pyrolytic graphite and thermally exfoliated graphite consisted of planar sheets. This effect was reproduced in behavior of dielectric functions of rippled graphite models calculated within the random phase approximation. We conclude that the energy of electron excitations in graphitic materials could be precisely tuned by a simple bending of hexagonal network without change of topology. Moreover, our investigation suggests that in such materials optical exciton can transfer energy to plasmon non-radiatively.

Sedelnikova, O. V., E-mail: o.sedelnikova@gmail.com; Bulusheva, L. G.; Okotrub, A. V. [Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Science, 3 Acad. Lavrentiev Ave., Novosibirsk 630090 (Russian Federation); Novosibirsk State University, 2 Pirogov Str., Novosibirsk 630090 (Russian Federation); Tomsk State University, 36 Lenina Ave., Tomsk 634050 (Russian Federation); Asanov, I. P. [Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Science, 3 Acad. Lavrentiev Ave., Novosibirsk 630090 (Russian Federation); Novosibirsk State University, 2 Pirogov Str., Novosibirsk 630090 (Russian Federation); Yushina, I. V. [Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Science, 3 Acad. Lavrentiev Ave., Novosibirsk 630090 (Russian Federation)

2014-04-21

350

Hot-electron-mediated desorption rates calculated from excited-state potential energy surfaces  

DEFF Research Database (Denmark)

We present a model for desorption induced by (multiple) electronic transitions [DIET (DIMET)] based on potential energy surfaces calculated with the delta self-consistent field extension of density-functional theory. We calculate potential energy surfaces of CO and NO molecules adsorbed on various transition-metal surfaces and show that classical nuclear dynamics does not suffice for propagation in the excited state. We present a simple Hamiltonian describing the system with parameters obtained from the excited-state potential energy surface and show that this model can describe desorption dynamics in both the DIET and DIMET regimes and reproduce the power-law behavior observed experimentally. We observe that the internal stretch degree of freedom in the molecules is crucial for the energy transfer between the hot electrons and the molecule when the coupling to the surface is strong.

Olsen, Thomas; Gavnholt, Jeppe

2009-01-01

351

Electron energy distributions and excitation rates in high-frequency argon discharges  

International Nuclear Information System (INIS)

The electron energy distribution functions and rate coefficients for excitation and ionisation in argon under the action of an uniform high-frequency electric field were calculated by numerically solving the homogeneous Boltzmann equation. Analytic calculations in the limiting cases ?>>?sub(c) and ?<electron-neutral collision frequency for momentum transfer, are also presented and shown to be in very good agreement with the numerical computations. The results reported here are relevant for the modelling of high-frequency discharges in argon and, in particular, for improving recent theoretical descriptions of a plasma column sustained by surface microwaves. The properties of surface wave produced plasmas make them interesting as possible substitutes for other more conventional plasma sources for such important applications as plasma chemistry laser excitation, plasma etching spectroscopic sources etc...

352

Hot electron mediated desorption rates calculated from excited state potential energy surfaces  

CERN Document Server

We present a model for Desorption Induce by (Multiple) Electronic Transitions (DIET/DIMET) based on potential energy surfaces calculated with the Delta Self-Consistent Field extension of Density Functional Theory. We calculate potential energy surfaces of CO and NO molecules adsorbed on various transition metal surfaces, and show that classical nuclear dynamics does not suffice for propagation in the excited state. We present a simple Hamiltonian describing the system, with parameters obtained from the excited state potential energy surface, and show that this model can describe desorption dynamics in both the DIET and DIMET regime, and reproduce the power law behavior observed experimentally. We observe that the internal stretch degree of freedom in the molecules is crucial for the energy transfer between the hot electrons and the molecule when the coupling to the surface is strong.

Olsen, Thomas; Schiøtz, Jakob

2008-01-01

353

Scintillation mechanisms and limiting factors on each step of relaxation of electronic excitations  

International Nuclear Information System (INIS)

The scintillation mechanisms in inorganic crystalline media are described in the general case in two special interest cases: rare earth containing crystals and crosse-luminescent crystals. The consideration of the energy relaxation in an insulating material is based on the band theory. The energy relaxation in scintillators consists of the three main steps: the creation of electronic excitations, the transfer to luminescent centers and the emission of luminescent centers. The simulation of the energy relaxation is performed using the Monte Carlo technique which describes the thermalization, the separated diffusion and the spatial correlation of electronic excitations. The factors limiting the performance of scintillators are identified as well as their effects on the efficiency and the energy resolution of inorganic scintillators are considered at the each step of the energy relaxation

354

Degradation of Eu(fod)3 electronic excitation energy in benzene, toluene, and o-xylene solutions  

International Nuclear Information System (INIS)

Ways of energy dissipation resulting from Eu(fod)3 electronic excitation in homological series of solvents-benzene, toluene and o-xylene have been studied. It is ascertained that probability of the electronic excitation energy degradation increases with an increase in the number of methyl groups at aromatic ring. The values of the activation energy of Eu(fod)3 chelate excited state quenching, which correlate with the number of methyl groups in the series benzene (35.6±1.3 kJ/mol), toluene (39.3±1.3 kJ/mol), o-xylene (54.4±0.8 kJ/mol), have been determined. The results obtained suggest that the main contribution to Eu(3+) electronic excitation energy degradation in homological series benzene, toluene, o-xylene is made by nonradiating energy transfer to high-frequency overtones of oscillations in the solvent C-H groups

355

Electron densities and the excitation of CN in molecular clouds  

Science.gov (United States)

In molecular clouds of modest density and relatively high fractional ionization, the rotational excitation of CN is controlled by a competition among electron impact, neutral impact and the interaction with the cosmic background radiation. The degree of excitation can be measured through optical absorption lines and millimeter-wave emission lines. The available, accurate data on CN in diffuse and translucent molecular clouds are assembled and used to determine electron densities. The derived values, n(e) = roughly 0.02 - 0.5/cu cm, imply modest neutral densities, which generally agree well with determinations by other techniques. The absorption- and emission-line measurements of CN both exclude densities higher than n(H2) = roughly 10 exp 3.5/cu cm on scales varying from 0.001 to 60 arcsec in these clouds.

Black, John H.; Van Dishoeck, Ewine F.

1991-01-01

356

Rotational excitation of molecular nitrogen by electron impact  

International Nuclear Information System (INIS)

Collision cross sections for rotational excitation processes of a molecular nitrogen by electron impact are calculated at the energy range of 0.01 to 3 eV. It is shown that integrated cross sections for rotationally inelastic scattering are noticeably enhanced by the short range interaction between the scattered electron and the target nitrogen molecule at the impact energy as low as 0.08 eV. At the energy region of the well-known ?g resonance, it is confirmed that the cross section for the rotational excitation of j = 0 to j' = 4 becomes larger than that for j = 0 to j' = 2 by more than the ratio of statistical weight (2j' + 1) of the final rotational state j'. An explanation to this fact is given by analysing the expression of the integrated cross section. (author)

357

A study of vibrational relaxation of electronically-excited molecules  

International Nuclear Information System (INIS)

The time kinetics of the vibrational relaxation of excimers is studied in the diffusional approximation. Simple formulae for functions of nonstationary vibrational distribution are found for the electronically excited molecules. Some spectral-kinetic dependencies of the excimer luminescence are explained in a new way. The possibilities of the determination of excimer parameters are discussed. The dependence of energetical characteristics of excimer lasers on these parameters is particularly emphasized. (author). 22 refs, 5 figs

358

Raman scattering by electron-hole excitations in silver nanocrystals  

CERN Document Server

Raman scattering experiments from silver nanocrystals embedded in films of amorphous silica are reported. In addition to the low-frequency peak due to vibrational quadrupolar modes, a broadband is observed in the high-frequency range, with a maximum at about 1000 $cm^{-1}$. The linear dependence of the position of this maximum on the inverse cluster radius is in agreement with the Raman scattering by single or collective electron-hole excitations.

Portales, H; Saviot, L; Fujii, M; Sumitomo, M; Hayashi, S

2001-01-01

359

Long-Range Electron Transfer and Electronic Transport Through Macromolecules  

CERN Document Server

A theory of electrical transport through molecular wires is used to estimate the electronic factor in the intramolecular electron transfer (ET) in porphyrin-nitrobenzene supermolecules, and to analyze its structure. The chosen molecules have complex donor and acceptor configurations, and relatively simple structure of the bridge, which enables us to concentrate our studies on the donor/acceptor coupling to the bridge. We present analytical and numerical results concerning the effect of donor/acceptor coupling to the bridge on the ET process in molecules with complex donor/acceptor subsystems. PACS 05.60.Gg, 36.20.-r

Zimbovskaya, N A; Zimbovskaya, Natalya; Gumbs, Godfrey

2002-01-01

360

High resolution IR diode laser study of collisional energy transfer between highly vibrationally excited monofluorobenzene and CO2: the effect of donor fluorination on strong collision energy transfer.  

Science.gov (United States)

Collisional energy transfer between vibrational ground state CO2 and highly vibrationally excited monofluorobenzene (MFB) was studied using narrow bandwidth (0.0003 cm(-1)) IR diode laser absorption spectroscopy. Highly vibrationally excited MFB with E' = ?41,000 cm(-1) was prepared by 248 nm UV excitation followed by rapid radiationless internal conversion to the electronic ground state (S1?S0*). The amount of vibrational energy transferred from hot MFB into rotations and translations of CO2 via collisions was measured by probing the scattered CO2 using the IR diode laser. The absolute state specific energy transfer rate constants and scattering probabilities for single collisions between hot MFB and CO2 were measured and used to determine the energy transfer probability distribution function, P(E,E'), in the large ?E region. P(E,E') was then fit to a bi-exponential function and extrapolated to the low ?E region. P(E,E') and the biexponential fit data were used to determine the partitioning between weak and strong collisions as well as investigate molecular properties responsible for large collisional energy transfer events. Fermi's Golden rule was used to model the shape of P(E,E') and identify which donor vibrational motions are primarily responsible for energy transfer. In general, the results suggest that low-frequency MFB vibrational modes are primarily responsible for strong collisions, and govern the shape and magnitude of P(E,E'). Where deviations from this general trend occur, vibrational modes with large negative anharmonicity constants are more efficient energy gateways than modes with similar frequency, while vibrational modes with large positive anharmonicity constants are less efficient at energy transfer than modes of similar frequency. PMID:25527934

Kim, Kilyoung; Johnson, Alan M; Powell, Amber L; Mitchell, Deborah G; Sevy, Eric T

2014-12-21

361

Lowest Energy Electronic Transition in Aqueous Cl- Salts: Cl-?(H2O)6 Charge Transfer Transition  

OpenAIRE

We use UV resonance Raman spectroscopy to probe the lowest energy allowed electronic transitions of aqueous solutions containing Cl- salts. We show that the waters hydrating the Cl- are involved in charge transfer transitions that transfer electron density from Cl- to the water molecules. These charge transfer transitions cause significant change in the H-O-H bond angle in the excited states, which results in a strong enhancement of the preresonance Raman intensity of the water bending modes....

Xiong, Kan; Asher, Sanford A.

2011-01-01

362

Theory of nuclear excitation by electron capture for heavy ions  

International Nuclear Information System (INIS)

The resonant process of nuclear excitation by electron capture (NEEC) in collisions involving highly-charged ions has been investigated theoretically. NEEC is a rare recombination process in which a free electron is captured into a bound shell of an ion with the simultaneous excitation of the nucleus. Total cross sections for NEEC followed by the radiative decay of the excited nucleus are presented for various collision systems. The possibility to observe the NEEC in scattering experiments with trapped or stored ions was discussed focusing on the cases with the largest calculated resonance strength. As the photons emitted in different channels of the electron recombination process are indistinguishable in the total cross section, the interference between NEEC followed by the radiative decay of the nucleus and radiative recombination was investigated. The angular distribution of the emitted photons in the recombination process provides means to discern the two processes. Angular differential cross sections for the emitted photons in the case of E2 nuclear transitions were presented for several heavy elements. (orig.)

363

Theory of nuclear excitation by electron capture for heavy ions  

Energy Technology Data Exchange (ETDEWEB)

The resonant process of nuclear excitation by electron capture (NEEC) in collisions involving highly-charged ions has been investigated theoretically. NEEC is a rare recombination process in which a free electron is captured into a bound shell of an ion with the simultaneous excitation of the nucleus. Total cross sections for NEEC followed by the radiative decay of the excited nucleus are presented for various collision systems. The possibility to observe the NEEC in scattering experiments with trapped or stored ions was discussed focusing on the cases with the largest calculated resonance strength. As the photons emitted in different channels of the electron recombination process are indistinguishable in the total cross section, the interference between NEEC followed by the radiative decay of the nucleus and radiative recombination was investigated. The angular distribution of the emitted photons in the recombination process provides means to discern the two processes. Angular differential cross sections for the emitted photons in the case of E2 nuclear transitions were presented for several heavy elements. (orig.)

Gagyi-Palffy, A.

2006-07-01

364

Electronic excitation energies of ZniSi nanoparticles  

International Nuclear Information System (INIS)

The excitation energies of small ZnS nanoclusters characterized in previous studies have been calculated using TDDFT. The relativistic pseudopotentials of Stevens et al have been used, including Zn 4s2 electrons and S 3s2 and 3p4 electrons as valence electrons. Results obtained with these pseudopotentials are compared to those obtained considering also Zn 3s23p63d10 electrons in the valence part, and demonstrated to be consistent. The results show that spheroid-like bubble structures have absorption energies in the range of 5-5.3 eV for small sizes, which decreases to 5 eV with increasing particle size

365

Many-body approach to electronic excitations concepts and applications  

CERN Document Server

The many-body-theoretical basis and applications of theoretical spectroscopy of condensed matter, e.g. crystals, nanosystems, and molecules are unified in one advanced text for readers from graduate students to active researchers in the field. The theory is developed from first principles including fully the electron-electron interaction and spin interactions. It is based on the many-body perturbation theory, a quantum-field-theoretical description, and Green's functions. The important expressions for ground states as well as electronic single-particle and pair excitations are explained. Based on single-particle and two-particle Green's functions, the Dyson and Bethe-Salpeter equations are derived. They are applied to calculate spectral and response functions. Important spectra are those which can be measured using photoemission/inverse photoemission, optical spectroscopy, and electron energy loss/inelastic X-ray spectroscopy. Important approximations are derived and discussed in the light of selected computa...

Bechstedt, Friedhelm

2015-01-01

366

Energy transfer in aminonaphthalimide-boron-dipyrromethene (BODIPY) dyads upon one- and two-photon excitation: applications for cellular imaging.  

Science.gov (United States)

Aminonaphthalimide-BODIPY energy transfer cassettes were found to show very fast (kEET ? 10(10)-10(11) s(-1) and efficient BODIPY fluorescence sensitization. This was observed upon one- and two-photon excitation, which extends the application range of the investigated bichromophoric dyads in terms of accessible excitation wavelengths. In comparison with the direct excitation of the BODIPY chromophore, the two-photon absorption cross-section ? of the dyads is significantly incremented by the presence of the aminonaphthalimide donor [? ? 10 GM for the BODIPY versus 19-26 GM in the dyad at ?(exc)=840 nm; 1 GM (Goeppert-Mayer unit)=10(-50) cm(4) smolecule(-1) photon-(1)]. The electronic decoupling of the donor and acceptor, which is a precondition for the energy transfercassette concept, was demonstrated by time-dependent density functional theory calculations. The applicability of the new probes in the one- and twophoton excitation mode was demonstrated in a proof-of-principle approach in the fluorescence imaging of HeLa cells. To the best of our knowledge, this is the first demonstration of the merging of multiphoton excitation with the energy transfer cassette concept for a BODIPY-containing dyad. PMID:24730057

Collado, Daniel; Remón, Patricia; Vida, Yolanda; Najera, Francisco; Sen, Pratik; Pischel, Uwe; Perez-Inestrosa, Ezequiel

2014-03-01

367

Energy Transfer between Gd3+ and Other Rare-earth Ions in GdPO4 under Vacuum UV Excitation  

Science.gov (United States)

Under 172 nm light excitation, energy transfer from Pr3+, Nd3+, Tb3+ or Tm3+ to Gd3+ resulting in strong Gd3+ emission at 312 nm was observed in GdPO4 doped with one of these ions. Among them, Pr3+ provides the strongest Gd3+ emission and Tm3+ is the second best. A wide energy gap between the 5d state and 4f levels of Pr3+ resulting from a simple electron configuration of 4f2 leads to energy transfer from a 5d-to-4f transition of Pr3+ to a 4f-to-4f absorption of Gd3+. Presumably, this is also the case with Tm3+, which has an electron configuration similar to that of Pr3+. The quantum output of the Gd3+ emission from GdPO4:Pr3+ (1 mol.%) relative to the blue luminescence of BaMgAl10O17:Eu2+, a commercial phosphor for plasma display panels, is about 37% under 172 nm excitation. Reverse energy transfer from Gd3+ to Dy3+ occurs under excitation by the Gd3+ absorption at 276 nm.

Sato, Yuji; Kumagai, Takashi; Okamoto, Shinji; Yamamoto, Hajime; Kunimoto, Takashi

2004-06-01

368

Electronic excitations from a linear-response range-separated hybrid scheme  

CERN Document Server

We study linear-response time-dependent density-functional theory (DFT) based on the single-determinant range-separated hybrid (RSH) scheme, i.e. combining a long-range Hartree-Fock exchange kernel with a short-range DFT exchange-correlation kernel, for calculating electronic excitation energies of molecular systems. It is an alternative to the long-range correction (LC) scheme which has a standard full-range DFT correlation kernel instead of only a short-range one. We discuss the local-density approximation (LDA) to the short-range exchange-correlation kernel, and assess the performance of the linear-response RSH scheme for singlet-singlet and singlet-triplet valence and Rydberg excitations in the N2, CO, H2CO, C2H4, and C6H6 molecules, and for the first charge-transfer excitation in the C2H4-C2F4 dimer. The introduction of long-range HF exchange corrects the underestimation of charge-transfer and high-lying Rydberg excitation energies obtained with standard (semi)local density-functional approximations, but...

Rebolini, Elisa; Toulouse, Julien

2013-01-01

369

Electronic excitations and defects in fluoroperovskite LiBaF3  

Science.gov (United States)

A survey of the present situation with respect to knowledge of lattice defects, electronic excitations, such as excitons and localized excitons, as well as energy storage and transfer phenomena in LiBaF3 crystals is given. Both phenomenological models and experimental interpretations of optical absorption bands, tentatively associated with F-type (electron) centers created by X-ray or electron irradiation, is reviewed. Interpretation of three radiative processes (super-fast core-valence transitions, slow trapped exciton luminescence and luminescence of structure defects) observed in undoped LiBaF3 crystals is analyzed with respect to practical application. Attention is paid to the behavior of ultraviolet emission so far ascribed to self-trapped exciton luminescence and also observed as a result of electron recombination with localized hole at various temperatures (even at room temperature), depending on crystal purity and growth conditions. Finally, some aspects of ionic processes in thermal relaxation of defects are pointed to.

Springis, Maris; Brikmane, Liga; Tale, Ivar; Kulis, Peteris

2003-08-01

370

The reaction dynamics of alkali dimer molecules and electronically excited alkali atoms with simple molecules  

Energy Technology Data Exchange (ETDEWEB)

This dissertation presents the results from the crossed molecular beam studies on the dynamics of bimolecular collisions in the gas phase. The primary subjects include the interactions of alkali dimer molecules with simple molecules, and the inelastic scattering of electronically excited alkali atoms with O{sub 2}. The reaction of the sodium dimers with oxygen molecules is described in Chapter 2. Two reaction pathways were observed for this four-center molecule-molecule reaction, i.e. the formations of NaO{sub 2} + Na and NaO + NaO. NaO{sub 2} products exhibit a very anisotropic angular distribution, indicating a direct spectator stripping mechanism for this reaction channel. The NaO formation follows the bond breaking of O{sub 2}, which is likely a result of a charge transfer from Na{sub 2} to the excited state orbital of O{sub 2}{sup {minus}}. The scattering of sodium dimers from ammonium and methanol produced novel molecules, NaNH{sub 3} and Na(CH{sub 3}OH), respectively. These experimental observations, as well as the discussions on the reaction dynamics and the chemical bonding within these molecules, will be presented in Chapter 3. The lower limits for the bond dissociation energies of these molecules are also obtained. Finally, Chapter 4 describes the energy transfer between oxygen molecules and electronically excited sodium atoms.

Hou, H. [Univ. of California, Berkeley, CA (United States). Dept. of Chemistry]|[Lawrence Berkeley National Lab., CA (United States). Chemical Sciences Div.

1995-12-01

371

The role of electronic excitation in cold atom-ion chemistry  

OpenAIRE

The role of electronic excitation in charge exchange chemical reactions between ultracold Ca atoms and Ba$^+$ ions, confined in a hybrid trap, is studied. This prototypical system is energetically precluded from reacting in its ground state, allowing a particularly simple interpretation of the influence of electronic excitation. It is found that while electronic excitation of the ion can critically influence the chemical reaction rate, electronic excitation of the neutral at...

Sullivan, Scott T.; Rellergert, Wade G.; Kotochigova, Svetlana; Hudson, Eric R.

2012-01-01

372

State preparation and excited electronic and vibrational behavior in hemes.  

Science.gov (United States)

The temporally overlapping, ultrafast electronic and vibrational dynamics of a model five-coordinate, high-spin heme in a nominally isotropic solvent environment has been studied for the first time with three complementary ultrafast techniques: transient absorption, time-resolved resonance Raman Stokes, and time-resolved resonance Raman anti-Stokes spectroscopies. Vibrational dynamics associated with an evolving ground-state species dominate the observations. Excitation into the blue side of the Soret band led to very rapid S2 --> S1 decay (sub-100 fs), followed by somewhat slower (800 fs) S1 --> S0 nonradiative decay. The initial vibrationally excited, non-Boltzmann S0 state was modeled as shifted to lower energy by 300 cm(-1) and broadened by 20%. On a approximately 10 ps time scale, the S0 state evolved into its room-temperature, thermal distribution S0 profile largely through VER. Anti-Stokes signals disappear very rapidly, indicating that the vibrational energy redistributes internally in about 1-3 ps from the initial accepting modes associated with S1 --> S0 internal conversion to the rest of the macrocycle. Comparisons of anti-Stokes mode intensities and lifetimes from TRARRS studies in which the initial excited state was prepared by ligand photolysis [Mizutani, T.; Kitagawa, T. Science 1997, 278, 443, and Chem. Rec. 2001, 1, 258] suggest that, while transient absorption studies appear to be relatively insensitive to initial preparation of the electronic excited state, the subsequent vibrational dynamics are not. Direct, time-resolved evaluation of vibrational lifetimes provides insight into fast internal conversion in hemes and the pathways of subsequent vibrational energy flow in the ground state. The overall similarity of the model heme electronic dynamics to those of biological systems may be a sign that the protein's influence upon the dynamics of the heme active site is rather subtle. PMID:17020382

Challa, J Reddy; Gunaratne, Tissa C; Simpson, M Cather

2006-10-12

373

Review of electron impact excitation cross sections for copper atom  

International Nuclear Information System (INIS)

Excitation of atomic copper by electron impact plays an important role in the copper vapor laser and accurate cross sections are needed for understanding and modeling laser performance. During the past seven years, there have been several attempts to normalize the relative elastic and inelastic cross sections measured by Trajmar and coworkers. However, each of these efforts have yielded different cross sections, and the uncertainty in the correct normalization of the data has been a source of confusion and concern for the kinetic modeling efforts. This difficulty has motivated us to review previous work on the electron impact excitation of copper atom and to perform new calculations of the inelastic cross sections using the impact parameter method. In this memorandum we review the previous attempts to normalize the experimental data and provide a critical assessment of the accuracy of the resulting cross sections. We also present new theoretical cross sections for the electron impact excitation of the 2S ? 2P0 and 2S ? 2D transitions in copper. When the experimental cross sections are renormalized to the results of the impact parameter calculations, they are a factor of three smaller than those published in the latest paper of Trajmar et. al. At impact energies above 60 eV the excitation cross sections obtained with the impact parameter method agree well with the results of the very recent, unpublished, close-coupling calculations of Henry. This agreement suggests that the present normalization of the experimental cross sections is probably the most reliable one obtained to date

374

Photoinduced electron transfer reactions of ruthenium(II)-complexes containing amino acid with quinones.  

Science.gov (United States)

With the aim of mimicking, at basic level the photoinduced electron transfer process in the reaction center of photosystem II, ruthenium(II)-polypyridyl complexes, carrying amino acids were synthesized and studied their photoinduced electron transfer reactions with quinones by steady state and time resolved measurements. The reaction of quinones with excited state of ruthenium(II)-complexes, I-V in acetonitrile has been studied by luminescence quenching technique and the rate constant, k(q), values are close to the diffusion controlled rate. The detection of the semiquinone anion radical in this system using time-resolved transient absorption spectroscopy confirms the electron transfer nature of the reaction. The semiclassical theory of electron transfer has been successfully applied to the photoluminescence quenching of Ru(II)-complexes with quinones. PMID:24590578

Eswaran, Rajkumar; Kalayar, Swarnalatha; Paulpandian, Muthu Mareeswaran; Seenivasan, Rajagopal

2014-05-01

375

Ultrafast Excited State Intramolecular Proton Transfer Dynamics of 1-Hydroxyanthraquinone in Solution  

International Nuclear Information System (INIS)

Proton transfer reaction is one of the most fundamental processes in chemistry and life science. Excited state intramolecular proton transfer (ESIPT) has been studied as a model system of the proton transfer, since it can be conveniently initiated by light. We report ESIPT reaction dynamic of 1-hydroxy-anthraquione (1-HAQ) in solution by highly time-resolved fluorescence. ESIPT time of 1-HAQ is determined to be 45 ± 10 fs directly from decay of the reactant fluorescence and rise of the product fluorescence. High time resolution allows observation of the coherent vibrational wave packet motion in the excited state of the reaction product tautomer. The coherently excited vibrational mode involves large displacement of the atoms, which shortens the distance between the proton donor and the acceptor. With the theoretical analysis, we propose that the ESIPT of 1-HAQ proceeds barrierlessly with assistance of the skeletal vibration, which in turn becomes excited coherently by the ESIPT reaction

376

Lower bound of energy dissipation in optical excitation transfer via optical near-field interactions.  

Science.gov (United States)

We theoretically analyzed the lower bound of energy dissipation required for optical excitation transfer from smaller quantum dots to larger ones via optical near-field interactions. The coherent interaction between two quantum dots via optical near-fields results in unidirectional excitation transfer by an energy dissipation process occurring in the larger dot. We investigated the lower bound of this energy dissipation, or the intersublevel energy difference at the larger dot, when the excitation appearing in the larger dot originated from the excitation transfer via optical near-field interactions. We demonstrate that the energy dissipation could be as low as 25 ?eV. Compared with the bit flip energy of an electrically wired device, this is about 10? times more energy efficient. The achievable integration density of nanophotonic devices is also analyzed based on the energy dissipation and the error ratio while assuming a Yukawa-type potential for the optical near-field interactions. PMID:21165087

Naruse, Makoto; Hori, Hirokazu; Kobayashi, Kiyoshi; Holmström, Petter; Thylén, Lars; Ohtsu, Motoichi

2010-11-01

377

Photoinduced electron transfer in ordered polymers  

Energy Technology Data Exchange (ETDEWEB)

The present report gives a brief account of the following elements of work related to photochemical electron transfer themes: (1) the synthesis and Photochemical characterization of chromophore-bound peptides and amino acid model compounds based on the amino acids, tryptophan and the spacer residue, alanine (Ala); (2) the study of binding of cationic organic dyes to a peptide electrolyte, for which cooperative dye loading and helix formation is important; (3) the completion of work on a new series of acridinium chromophores that have rod-like'' arrangements of linked aryl rings for assembly of electron donor-acceptor systems that exhibit long lived charge separation; and (4) use of the modified form of the peptide, poly-L-histidine, as a template for sulfide oxidation.

Jones, G. II.

1993-01-01

378

Energy transfer followed by electron transfer in a supramolecular triad composed of boron dipyrrin, zinc porphyrin, and fullerene: a model for the photosynthetic antenna-reaction center complex.  

Science.gov (United States)

The first example of a working model of the photosynthetic antenna-reaction center complex, constructed via self-assembled supramolecular methodology, is reported. For this, a supramolecular triad is assembled by axially coordinating imidazole-appended fulleropyrrolidine to the zinc center of a covalently linked zinc porphyrin-boron dipyrrin dyad. Selective excitation of the boron dipyrrin moiety in the boron dipyrrin-zinc porphyrin dyad resulted in efficient energy transfer (k(ENT)(singlet) = 9.2 x 10(9) s(-)(1); Phi(ENT)(singlet) = 0.83) creating singlet excited zinc porphyrin. Upon forming the supramolecular triad, the excited zinc porphyrin resulted in efficient electron transfer to the coordinated fullerenes, resulting in a charge-separated state (k(cs)(singlet) = 4.7 x 10(9) s(-)(1); Phi(CS)(singlet) = 0.9). The observed energy transfer followed by electron transfer in the present supramolecular triad mimics the events of natural photosynthesis. Here, the boron dipyrrin acts as antenna chlorophyll that absorbs light energy and transports spatially to the photosynthetic reaction center, while the electron transfer from the excited zinc porphyrin to fullerene mimics the primary events of the reaction center where conversion of the electronic excitation energy to chemical energy in the form of charge separation takes place. The important feature of the present model system is its relative "simplicity" because of the utilized supramolecular approach to mimic rather complex "combined antenna-reaction center" events of photosynthesis. PMID:15212538

D'Souza, Francis; Smith, Phillip M; Zandler, Melvin E; McCarty, Amy L; Itou, Mitsunari; Araki, Yasuyuki; Ito, Osamu

2004-06-30

379

Electronic excited States of polynucleotides: a study by electroabsorption spectroscopy.  

Science.gov (United States)

Electroabsorption spectra were obtained for single-stranded polynucleotides poly(U), poly(C), poly(A), and poly(G) in glycerol/water glass at low temperature, and the differences in permanent dipole moment (Deltamu) and polarizability (Deltaalpha) were estimated for several spectral ranges covering the lowest energy absorption band around 260 nm. In each spectral range, the electrooptical parameters associated with apparent features in the absorption spectrum exhibit distinct values representing either a dominant single transition or the resultant value for a group of a relatively narrow cluster of overlapping transitions. The estimated spacing in energy between electronic origins of these transitions is larger than the electronic coupling within the Coulombic interaction model which is usually adopted in computational studies. The electroabsorption data allow us to distinguish a weak electronic transition associated with a wing in polynucleotide absorption spectra, at an energy below the electronic origin in absorption spectra of monomeric nucleobases. In poly(C) and poly(G), these low-energy transitions are related to increased values of Deltamu and Deltaalpha, possibly indicating a weak involvement of charge resonance in the respective excited states. A model capable of explaining the origin of low-energy excited states, based on the interaction of pipi* and npi* transitions in neighboring bases, is introduced and briefly discussed on the grounds of point dipole interaction. PMID:17266277

Krawczyk, Stanislaw; Luchowski, Rafal

2007-02-01

380

Simulação Monte Carlo de mecanismo de transferência de energia de excitação eletrônica: modelo de Perrin para a supressão estática da luminescência / Monte Carlo simulation of electronic excitation energy transfer: Perrin´s model for static luminescence quenching  

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: Portuguese Abstract in portuguese [...] Abstract in english A software based in the Monte Carlo method has been developed aiming the teaching of the Perrin´s model for static luminescence quenching. This software allows the student to easily simulate the luminescence decays of emissive molecules in the presence of quenching ones. The software named PERRIN wa [...] s written for FreeBASIC compiler and it can be applied for systems where the molecules remain static during its excited state lifetime. The good agreement found between the simulations and the expected theoretical results shows that it can be used for the luminescence and excited states decay kinetic teaching.

João Batista Marques, Novo; Lauro Camargo, Dias Júnior.

381

Spectroscopy of highly excited vibrational states of HCN in its ground electronic state  

International Nuclear Information System (INIS)

An experimental technique based on a scheme of vibrationally mediated photodissociation has been developed and applied to the spectroscopic study of highly excited vibrational states in HCN, with energies between 29 000 and 30 000 cm-1. The technique consists of four sequential steps: in the first one, a high power laser is used to vibrationally excite the sample to an intermediate state, typically (0,0,4), the ?3 mode being approximately equivalent to the C-H stretching vibration. Then a second laser is used to search for transitions between this intermediate state and highly vibrationally excited states. When one of these transitions is found, HCN molecules are transferred to a highly excited vibrational state. Third, a ultraviolet laser photodissociates the highly excited molecules to produce H and CN radicals in its A 2? electronic state. Finally, a fourth laser (probe) detects the presence of the CN(A) photofragments by means of an A?B?X laser induced fluorescence scheme. The spectra obtained with this technique, consisting of several rotationally resolved vibrational bands, have been analyzed. The positions and rotational parameters of the states observed are presented and compared with the results of a state-of-the-art variational calculation

382

Electron transfer pathways in microbial oxygen biocathodes  

International Nuclear Information System (INIS)

The ability of some bacteria to enhance the rate of cathodic oxygen reduction to water has been recently discovered, opening the way to an entirely renewable and environmentally friendly concept of biocathode. In this study we reveal that several mechanisms may induce catalytic effects by bacteria. These comprise mechanisms that are putatively beneficial to the bacteria as well as mechanisms which are merely side effects, including quinone autoxidation and direct O2 reduction by heme compounds. Here we showed that 1 ?M of ACNQ is able to generate a significant catalytic wave for oxygen reduction, with onset at approximately 0 V vs. SHE. Similarly, adsorption of hemin on a carbon surface catalyses O2 reduction to H2O2 with an onset of +0.2 V vs. SHE. To evaluate the catalytic pathways of live cells on cathodic oxygen reduction, two species of electrochemically active bacteria were selected as pure cultures, namely Acinetobacter calcoaceticus and Shewanella putrefaciens. The former appears to exploit a self-excreted redox compound with redox characteristics matching those of pyrroloquinoline quinone (PQQ) for extracellular electron transfer. The latter appears to utilise outer membrane-bound redox compounds. Interaction of quinones and cytochromes with the membrane-bound electron transfer chain is yet to be proven.

383

Mediated Electron Transfer at Redox Active Monolayers  

Directory of Open Access Journals (Sweden)

Full Text Available A theoretical model describing the transport and kinetic processes involved in heterogeneous redox catalysis of solution phase reactants at electrode surfaces coated with redox active monolayers is presented. Although the analysis presented has quite general applicability, a specific focus of the paper is concerned with the idea that redox active monolayers can be used to model an ensemble of individual molecular nanoelectrodes. Three possible rate determining steps are considered: heterogeneous electron transfer between immobilized mediator and support electrode ; bimolecular chemical reaction between redox mediator and reactant species in the solution phase, and diffusional mass transport of reactant in solution. A general expression for the steady state reaction flux is derived which is valid for any degree of reversibility of both the heterogeneous electron transfer reaction involving immobilized mediator species and of the bimolecular cross exchange reaction between immobilized mediator and solution phase reactant. The influence of reactant transport in solution is also specifically considered. Simplified analytical expressions for the net reaction flux are derived for experimentally reasonable situations and a kinetic case diagram is constructed outlining the relationships between the various approximate solutions. The theory enables simple diagnostic plots to be constructed which can be used to analyse experimental data.

Michael E.G. Lyons

2001-12-01

384

Resonant vibrational excitation of methylamine by low energy electron impact  

International Nuclear Information System (INIS)

The study of the excitation functions of the NH2 and CH3 stretching modes of methylamine in the 4--12 eV electron impact energy range shows evidence of a broad shape resonance built on the electronic ground state and centered at 7.5 eV. This resonance is formed by the trapping of the incident electron in the second unoccupied molecular orbital and is of A double-prime symmetry. The elastic and inelastic angular differential cross sections measured in the 10 degree--90 degree range suggest a dominant p character. A second shape resonance located at higher energy contributes mainly to the enhancement of the CH3 deformation modes and is very likely of A' symmetry. A comparison is made between the resonances observed in NH3 and CH3NH2. The effect of the methyl substitution is briefly discussed

385

Dissociative electron attachment and vibrational excitation of the chlorine molecule  

International Nuclear Information System (INIS)

This paper is aimed at the theoretical investigation of the inelastic processes taking place in resonant collisions of low-energy electrons with the chlorine molecule. Dissociative electron attachment and vibrational excitation of Cl2 by electron impact is investigated in the energy range 0-1.5 eV, where the 2?u+ resonance plays the central role. The calculations were carried out within the framework of the nonlocal resonance model. This approach makes it possible to calculate the integrated cross sections of the above-mentioned processes for a variety of initial and final rovibrational states of the target molecule. The present model is constructed on the basis of ab initio fixed-nuclei R-matrix calculations using the so-called Feshbach-Fano R-matrix method. The Schwinger-Lanczos algorithm was utilized to solve the Lippmann-Schwinger equation describing the motion of the nuclei

386

Quantum tunneling resonant electron transfer process in Lorentzian plasmas  

Energy Technology Data Exchange (ETDEWEB)

The quantum tunneling resonant electron transfer process between a positive ion and a neutral atom collision is investigated in nonthermal generalized Lorentzian plasmas. The result shows that the nonthermal effect enhances the resonant electron transfer cross section in Lorentzian plasmas. It is found that the nonthermal effect on the classical resonant electron transfer cross section is more significant than that on the quantum tunneling resonant charge transfer cross section. It is shown that the nonthermal effect on the resonant electron transfer cross section decreases with an increase of the Debye length. In addition, the nonthermal effect on the quantum tunneling resonant electron transfer cross section decreases with increasing collision energy. The variation of nonthermal and plasma shielding effects on the quantum tunneling resonant electron transfer process is also discussed.

Hong, Woo-Pyo [Department of Electronics Engineering, Catholic University of Daegu, Hayang 712-702 (Korea, Republic of); Jung, Young-Dae, E-mail: ydjung@hanyang.ac.kr [Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180-3590 (United States); Department of Applied Physics and Department of Bionanotechnology, Hanyang University, Ansan, Kyunggi-Do 426-791 (Korea, Republic of)

2014-08-15

387

Cross sections for electron capture and excitation in collisions of Liq+ (q=1, 2, 3) with atomic hydrogen  

International Nuclear Information System (INIS)

The two-center atomic orbital close-coupling method is employed to study electron capture and excitation reactions in collisions of Liq+ (q?=?1-3) ions with ground state atomic hydrogen in the ion energy range from 0.1?keV/u to 300?keV/u, where u is the atomic mass unit. The interaction of the active electron with the projectile ions (Li+, Li2+) is represented by a model potential. Total and state-selective cross sections for charge transfer and excitation processes are calculated and compared with data from other sources when available

388

A stochastic reorganizational bath model for electronic energy transfer.  

Science.gov (United States)

Environmentally induced fluctuations of the optical gap play a crucial role in electronic energy transfer dynamics. One of the simplest approaches to incorporate such fluctuations in energy transfer dynamics is the well known Haken-Strobl-Reineker (HSR) model, in which the energy-gap fluctuation is approximated as white noise. Recently, several groups have employed molecular dynamics simulations and excited-state calculations in conjunction to account for excitation energies' thermal fluctuations. On the other hand, since the original work of HSR, many groups have employed stochastic models to simulate the same transfer dynamics. Here, we discuss a rigorous connection between the stochastic and the atomistic bath models. If the phonon bath is treated classically, time evolution of the exciton-phonon system can be described by Ehrenfest dynamics. To establish the relationship between the stochastic and atomistic bath models, we employ a projection operator technique to derive the generalized Langevin equations for the energy-gap fluctuations. The stochastic bath model can be obtained as an approximation of the atomistic Ehrenfest equations via the generalized Langevin approach. Based on this connection, we propose a novel scheme to take account of reorganization effects within the framework of stochastic models. The proposed scheme provides a better description of the population dynamics especially in the regime of strong exciton-phonon coupling. Finally, we discuss the effect of the bath reorganization in the absorption and fluorescence spectra of ideal J-aggregates in terms of the Stokes shifts. We find a simple expression that relates the reorganization contribution to the Stokes shifts - the reorganization shift - to the ideal or non-ideal exciton delocalization in a J-aggregate. The reorganization shift can be described by three parameters: the monomer reorganization energy, the relaxation time of the optical gap, and the exciton delocalization length. This simple relationship allows one to understand the physical origin of the Stokes shifts in molecular aggregates. PMID:24985614

Fujita, Takatoshi; Huh, Joonsuk; Aspuru-Guzik, Alán

2014-06-28

389

A stochastic reorganizational bath model for electronic energy transfer  

International Nuclear Information System (INIS)

Environmentally induced fluctuations of the optical gap play a crucial role in electronic energy transfer dynamics. One of the simplest approaches to incorporate such fluctuations in energy transfer dynamics is the well known Haken-Strobl-Reineker (HSR) model, in which the energy-gap fluctuation is approximated as white noise. Recently, several groups have employed molecular dynamics simulations and excited-state calculations in conjunction to account for excitation energies’ thermal fluctuations. On the other hand, since the original work of HSR, many groups have employed stochastic models to simulate the same transfer dynamics. Here, we discuss a rigorous connection between the stochastic and the atomistic bath models. If the phonon bath is treated classically, time evolution of the exciton-phonon system can be described by Ehrenfest dynamics. To establish the relationship between the stochastic and atomistic bath models, we employ a projection operator technique to derive the generalized Langevin equations for the energy-gap fluctuations. The stochastic bath model can be obtained as an approximation of the atomistic Ehrenfest equations via the generalized Langevin approach. Based on this connection, we propose a novel scheme to take account of reorganization effects within the framework of stochastic models. The proposed scheme provides a better description of the population dynamics especially in the regime of strong exciton-phonon coupling. Finally, we discuss the effect of the bath reorganization in the absorption and fluorescence spectra of ideal J-aggregates in terms of the Stokes shifts. We find a simple expression that relates the reorganization contribution to the Stokes shifts – the reorganization shift – to the ideal or non-ideal exciton delocalization in a J-aggregate. The reorganization shift can be described by three parameters: the monomer reorganization energy, the relaxation time of the optical gap, and the exciton delocalization length. This simple relationship allows one to understand the physical origin of the Stokes shifts in molecular aggregates

390

Transfer line TT70 (electrons from PS to SPS)  

CERN Multimedia

As injectors for LEP, PS and SPS had to be converted to the acceleration of electrons and positrons. So far, only positively charged particles had been transferred from the PS to the SPS, for the negatively charged electrons a new transfer line, TT70, had to be built. Due to the difference in level of the two machines, the transfer line slopes and tilts.

1981-01-01

391

Electrostatic mode coupling of beat-excited electron plasma waves  

International Nuclear Information System (INIS)

The process of beat excitation of electron plasma waves in a plasma containing a density ripple is studied theoretically, experimentally, and computationally. A single theoretical model has been developed which, for modest experimental parameters, predicts a new beat wave saturation mechanism. This mechanism involves the excitation of a spectrum of secondary electrostatic modes which divert pump energy from the beat wave and can lead to saturation of the beat wave at an amplitude well below that expected for relativistic detuning. Experiments designed to study the coupled mode spectrum were performed. The measured properties of the electrostatic spectrum are in reasonable agreement with the theory. To bridge the gap between the idealized model and the experiment, computer simulations were performed for a variety of parameters

392

Reaction of electron-transfer flavoprotein with electron-transfer flavoprotein-ubiquinone oxidoreductase  

International Nuclear Information System (INIS)

The oxidative half-reaction of electron-transfer flavoprotein (ETF), electron transfer from ETF to electron-transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO), is dependent on complementary surface charges on the two proteins. ETF is the positively charged member of the redox pair. The evidence is based on the pH and ionic strength dependencies of the comproportionation of oxidized ETF and ETF hydroquinone catalyzed by ETF-QO and on the effects of chemical modification of ETF on the comproportionation reaction. Acetylation of one and five epsilon-amino groups of lysyl residues results in 3- and 13-fold increases, respectively, in the K/sub m/ of ETF-QO for ETF but no change in V/sub max/. Amidination, which maintains positive charge at modified loci, has no effect on steady-state kinetic constants. These chemical modifications have no effect on the equilibrium constant for equilibration of ETF redox states. The K/sub m/ of ETF-QO for ETF is pH dependent above pH 8.5, suggesting titration of lysyl residues. The ionic strength dependence of TN/KmETF for the reaction follows the limiting Bronsted equation. The ETF-QO-catalyzed comproportionation reaction exhibits a primary deuterium isotope effect in D2O, perhaps indicating the participation of solvent water in the electron-transfer reaction

393

Interfacial Electron Transfer and Transient Photoconductivity Studied with Terahertz Spectroscopy  

Science.gov (United States)

Terahertz spectroscopy is distinguished from other far infrared and millimeter wave spectroscopies by its inherent phase sensitivity and sub-picosecond time resolution making it a versatile technique to study a wide range of physical phenomena. As THz spectroscopy is still a relatively new field, many aspects of THz generation mechanisms have not been fully examined. Using terahertz emission spectroscopy (TES), THz emission from ZnTe(110) was analyzed and found to be limited by two-photon absorption and free-carrier generation at high excitation fluences. Due to concerns about the continued use of fossil fuels, solar energy has been widely investigated as a promising source of renewable energy. Dye-sensitized solar cells (DSSCs) have been developed as a low-cost alternative to conventional photovoltaic solar cells. To solve the issues of the intermittency and inefficient transport associated with solar energy, researchers are attempting to adapt DSSCs for water oxidation and chemical fuel production. Both device designs incorporate sensitizer molecules covalently bound to metal oxide nanoparticles. The sensitizer, which is comprised of a chromophore and anchoring group, absorbs light and transfers an electron from its excited state to the conduction band of the metal oxide, producing an electric current. Using time-resolved THz spectroscopy (TRTS), an optical pump/THz probe technique, the efficiency and dynamics of electron injection from sensitizers to metal oxides was evaluated as a function of the chromophore, its anchoring group, and the metal oxide identity. Experiments for studying fully functioning DSSCs and water oxidation devices are also described. Bio-inspired pentafluorophenyl porphyrin chromophores have been designed and synthesized for use in photoelectrochemical water oxidation cells. Influences on the efficiency and dynamics of electron injection from the chromophores into TiO2 and SnO2 nanoparticles due to changes in both the central substituent to the porphyrin ring and degree of fluorination of ring substituents were analyzed. Due to the high reduction potentials of these sensitizers, injection into TiO2 was generally not observed. Injection timescales from the porphyrins into SnO2 depended strongly on the identity of the central substituent and were affected by competition with excited-state deactivation processes. The carboxylate anchoring group is commonly used to bind DSSC sensitizers to metal oxide surfaces but is typically not stable under the aqueous and oxidative conditions required for water oxidation. Electron injection efficiency and water stability of several alternative anchoring groups, including phosphonic acid, hydroxamic acid, acerylacetone, and boronic acid, were evaluated. While all of the anchoring groups exhibited water stability superior to carboxylate, the hydroxamate anchor had the best combination of ease of handling and electron injection efficiency. The effects on photoconductivity due to metal oxide morphology and the addition of dopants were also analyzed. Mixtures of anatase and rutile TiO 2 nanoparticles are known to exhibit cooperative effects which increase the efficiency of DSSCs and photocatalysis relative to the pure-phase materials. Through analysis of TRTS measurements, the mechanism of this synergistic effect was found to involve electron transfer from the lower-mobility, higher surface area rutile nanoparticles to anatase particles, resulting in a higher charge collection efficiency. In addition to morphology, doping has been investigated as a means of expanding the spectral range of visible absorption of photocatalysts. Doping ZnO nanowires with manganese(II) was found to significantly decrease the electron mobility, and doping with cobalt(II) increased the timescale for electron trapping. These differences can be understood by considering the changes to the band structure of ZnO effected by the dopants. Preliminary analyses of the solvent and electrolyte dependence on the electron injection rate and efficiency suggest that electron injection can be affected by

Milot, Rebecca Lee

394

Electron hole pair mediated vibrational excitation in CO scattering from Au(111): Incidence energy and surface temperature dependence  

Science.gov (United States)

We investigated the translational incidence energy (Ei) and surface temperature (Ts) dependence of CO vibrational excitation upon scattering from a clean Au(111) surface. We report absolute v = 0 ? 1 excitation probabilities for Ei between 0.16 and 0.84 eV and Ts between 473 and 973 K. This is now only the second collision system where such comprehensive measurements are available - the first is NO on Au(111). For CO on Au(111), vibrational excitation occurs via direct inelastic scattering through electron hole pair mediated energy transfer - it is enhanced by incidence translation and the electronically non-adiabatic coupling is about 5 times weaker than in NO scattering from Au(111). Vibrational excitation via the trapping desorption channel dominates at Ei = 0.16 eV and quickly disappears at higher Ei.

Shirhatti, Pranav R.; Werdecker, Jörn; Golibrzuch, Kai; Wodtke, Alec M.; Bartels, Christof

2014-09-01

395

Effect of electronic excitation on high-temperature flows behind strong shock waves  

Science.gov (United States)

In the present paper, a strongly non-equilibrium one-dimensional steady-state flow behind the plane shock wave is studied. We consider a high-temperature chemically reacting five-component ionized mixture of nitrogen species (N2/N22/N/N+/e-) taking into account electronic degrees of freedom in N and N+ (170 and 625 electronic energy levels respectively), and electronic-rotational-vibrational modes in N2 and N2+ (5 and 7 electronic terms). Non-equilibrium reactions of ionization, dissociation, recombination and charge-transfer are included to the kinetic scheme. The system of governing equations is written under the assumption that translation and internal energy relaxation is fast whereas chemical reactions and ionization proceed on the macroscopic gas-dynamics time-scale. The developed model is applied to simulate the flow behind a plane shock wave under initial conditions characteristic for the spacecraft re-entry from an interplanetary flight (Hermes and Fire II experiments). Fluid-dynamic parameters behind the shock wave as well as transport coefficients and the heat flux are calculated for the (N2/N2+/N/N+/e-) mixture. The effect of electronic excitation on kinetics, dynamics and heat transfer is analyzed. Whereas the contribution of electronic degrees of freedom to the flow macroparameters is negligible, their influence on the heat flux is found to be important under conditions of Hermes re-entry.

Istomin, V. A.; Kustova, E. V.

2014-12-01

396

Ultrafast photoinduced electron transfer in coumarin 343 sensitized TiO2-colloidal solution  

Directory of Open Access Journals (Sweden)

Full Text Available Photoinduced electron transfer from organic dye molecules to semiconductor nanoparticles is the first and most important reaction step for the mechanism in the so called “wet solar cells” [1]. The time scale between the photoexcitation of the dye and the electron injection into the conduction band of the semiconductor colloid varies from a few tens of femtoseconds to nanoseconds, depending on the specific electron transfer parameters of the system, e.g., electronic coupling or free energy values of donor and acceptor molecules [2–10]. We show that visible pump/ white light probe is a very efficient tool to investigate the electron injection reaction allowing to observe simultaneously the relaxation of the excited dye, the injection process of the electron, the cooling of the injected electron and the charge recombination reaction.

Michael Grätzel

1999-01-01

397

Electronically driven adsorbate excitation mechanism in femtosecond-pulse laser desorption  

DEFF Research Database (Denmark)

Femtosecond-pulse laser desorption is a process in which desorption is driven by a subpicosecond temperature pulse of order 5000 K in the substrate-adsorbate electron system, whose energy is transferred into the adsorbate center-of-mass degrees of freedom by a direct coupling mechanism. We present a systematic theoretical treatment of this coupling process in the language of an electronic friction, which generates Langevin noise in the adsorbate center-of-mass degrees of freedom, while the electronic degrees of freedom are at a high temperature. Starting from an influence-functional path-integral description, a simple formula for the electronic friction is defined which is valid at all electronic temperatures. At low temperatures the formalism makes contact with the electronic friction appearing in the theory of adsorbate vibrational damping, whereas at high temperatures comparable with the adsorbate electronic excitation energies the friction becomes strongly temperature dependent due to dominance by virtualexcitations between different adsorbate potential energy surfaces. The former regime is related to the electronic friction model for the desorption process, and the latter to the desorption induced by multiple electronic transistions model for the process; the present formulation comprises both regimes. Desorption is calculated both by a simple quasianalytic Kramers rate approach, and by numerical solution to the Langevin equation. The magnitude of the desorbed fraction and the time scale for desorption are compared to experimental results.

Brandbyge, Mads; Hedegård, Per

1995-01-01

398

Computer simulation of electronic excitation in atomic collision cascades  

Energy Technology Data Exchange (ETDEWEB)

The impact of an keV atomic particle onto a solid surface initiates a complex sequence of collisions among target atoms in a near-surface region. The temporal and spatial evolution of this atomic collision cascade leads to the emission of particles from the surface - a process usually called sputtering. In modern surface analysis the so called SIMS technology uses the flux of sputtered particles as a source of information on the microscopical stoichiometric structure in the proximity of the bombarded surface spots. By laterally varying the bombarding spot on the surface, the entire target can be scanned and chemically analyzed. However, the particle detection, which bases upon deflection in electric fields, is limited to those species that leave the surface in an ionized state. Due to the fact that the ionized fraction of the total flux of sputtered atoms often only amounts to a few percent or even less, the detection is often hampered by rather low signals. Moreover, it is well known, that the ionization probability of emitted particles does not only depend on the elementary species, but also on the local environment from which a particle leaves the surface. Therefore, the measured signals for different sputtered species do not necessarily represent the stoichiometric composition of the sample. In the literature, this phenomenon is known as the Matrix Effect in SIMS. In order to circumvent this principal shortcoming of SIMS, the present thesis develops an alternative computer simulation concept, which treats the electronic energy losses of all moving atoms as excitation sources feeding energy into the electronic sub-system of the solid. The particle kinetics determining the excitation sources are delivered by classical molecular dynamics. The excitation energy calculations are combined with a diffusive transport model to describe the spread of excitation energy from the initial point of generation. Calculation results yield a space- and time-resolved excitation energy density profile E(r,t) within the volume affected by the atomic collision cascade.The distribution E(r,t) is then converted into an electron temperature, which in a further step can be utilized to calculate the ionization probabilities of sputtered atoms using published theory. (orig.)

Duvenbeck, A.

2007-04-05

399

Computer simulation of electronic excitation in atomic collision cascades  

International Nuclear Information System (INIS)

The impact of an keV atomic particle onto a solid surface initiates a complex sequence of collisions among target atoms in a near-surface region. The temporal and spatial evolution of this atomic collision cascade leads to the emission of particles from the surface - a process usually called sputtering. In modern surface analysis the so called SIMS technology uses the flux of sputtered particles as a source of information on the microscopical stoichiometric structure in the proximity of the bombarded surface spots. By laterally varying the bombarding spot on the surface, the entire target can be scanned and chemically analyzed. However, the particle detection, which bases upon deflection in electric fields, is limited to those species that leave the surface in an ionized state. Due to the fact that the ionized fraction of the total flux of sputtered atoms often only amounts to a few percent or even less, the detection is often hampered by rather low signals. Moreover, it is well known, that the ionization probability of emitted particles does not only depend on the elementary species, but also on the local environment from which a particle leaves the surface. Therefore, the measured signals for different sputtered species do not necessarily represent the stoichiometric composition of the sample. In the literature, this phenomenon is known as the Matrix Effect in SIMS. In order to circumvent this principal shortcoming of SIMS, the present thesis develops an alternative computer simulation concept, which treats the electronic energy losses of all moving atoms as excitation sources feeding energy into the electronic sub-system of the solid. The particle kinetics determining the excitation sources are delivered by classical molecular dynamics. The excitation energy calculations are combined with a diffusive transport model to describe the spread of excitation energy from the initial point of generation. Calculation results yield a space- and time-resolved excitation energy density profile E(r,t) within the volume affected by the atomic collision cascade.The distribution E(r,t) is then converted into an electron temperature, which in a further step can be utilized to calculate the ionization probabilities of sputtered atoms using published theory. (orig.)

400

Effect of Electronic Excitation on Thin Film Growth  

Energy Technology Data Exchange (ETDEWEB)

The effect of nanosecond pulsed laser excitation on surface diffusion during growth of Ge on Si(100) at 250 degrees C was studied. In Situ reflection high-energy electron diffraction (RHEED) was used to measure the surface diffusion coefficient while ex situ atomic force microscopy (AFM) was used to probe the structure and morphology of the grown quantum dots. The results show that laser excitation of the substrate increases the surface diffusion during growth of Ge on Si(100), changes the growth morphology, improves crystalline structure of the grown quantum dots, and decreases their size distribution. A purely electronic mechanism of enhanced surface diffusion of the deposited Ge is proposed. Ge quantum dots were grown on Si(100)-(2x1) by pulsed laser deposition at various substrate temperatures using a femtosecond Ti:sapphire laser. In-situ reflection high-energy electron diffraction and ex-situ atomic force microscopy were used to analyze the fim structure and morphology. The morphology of germanium islands on silicon was studied at differect coverages. The results show that femtosecond pulsed laser depositon reduces the minimum temperature for epitaxial growth of Ge quantum dots to ~280 degrees C, which is 120 degrees C lower then previously observed in nanosecond pulsed laser deposition and more than 200 degrees C lower than that reported for molecular beam epitaxy and chemical vapor deposition.

Elsayed-Ali, Hani E. [Old Dominion University

2011-01-31

401

Compilation of electron collision excitation cross sections for neutro argon  

International Nuclear Information System (INIS)

The present work presents a compilation and critical analysis of the available data on electron collision excitation cross sections for neutral Argon levels. This study includes: 1.- A detailed description in intermediate coupling for all the levels belonging the 20 configurations 3p''5 ns(n=4 to 12), np(n=4 to 8) and nd(n=3 to 8) of neutral Argon. 2.- Calculation of the electron collision excitation cross sections in Born and Born-Oppenheimer-Ochkur approximations for all the levels in the 14 configurations 3p''5 ns(n=4 to 7), np(n=4 to 7) and nd(n=3 to 8). 3.- Comparison and discussion of the compiled data. These are the experimental and theoretical values available from the literature, and those from this work. 4.- Analysis of the regularities and systematic behaviors in order to determine which values can be considered more reliable. It is show that the concept of one electron cross section results quite useful for this purpose. In some cases it has been possible to obtain in this way approximate analytical expressions interpolating the experimental data. 5.- All the experimental and theoretical values studied are graphically presented and compared. 6.- The last part of the work includes a listing of several general purpose programs for Atomic Physics calculations developed for this work. (Author)

402

Electronically excited rubidium atom in a helium cluster or film  

Science.gov (United States)

We present theoretical studies of helium droplets and films doped with one electronically excited rubidium atom Rb? (P2). Diffusion and path integral Monte Carlo approaches are used to investigate the energetics and the structure of clusters containing up to 14 helium atoms. The surface of large clusters is approximated by a helium film. The nonpair additive potential energy surface is modeled using a diatomic in molecule scheme. Calculations show that the stable structure of Rb?Hen consists of a seven helium atom ring centered at the rubidium, surrounded by a tirelike second solvation shell. A very different structure is obtained when performing a "vertical Monte Carlo transition." In this approach, a path integral Monte Carlo equilibration starts from the stable configuration of a rubidium atom in the electronic ground state adsorbed to the helium surface after switching to the electronically excited surface. In this case, Rb?Hen relaxes to a weakly bound metastable state in which Rb? sits in a shallow dimple. The interpretation of the results is consistent with the recent experimental observations [G. Auböck et al., Phys. Rev. Lett. 101, 035301 (2008)].

Leino, Markku; Viel, Alexandra; Zillich, Robert E.

2008-11-01

403

Production of excited electrons at TESLA and CLIC based e gamma colliders  

OpenAIRE

We analyze the potential of TESLA and CLIC based electron-photon colliders to search for excited spin-1/2 electrons. The production of excited electrons in the resonance channel through the electron-photon collision and their subsequent decays to leptons and electroweak gauge bosons are investigated. We study in detail the three signal channels of excited electrons and the corresponding backgrounds through the reactions e gamma --> e gamma, e gamma --> eZ and e gamma --> nu ...

Kirca, Z.; Cakir, O.; Aydin, Z. Z.

2002-01-01

404

An efficient implementation of the localized operator partitioning method for electronic energy transfer  

CERN Document Server

The localized operator partitioning method [Y. Khan and P. Brumer, J. Chem. Phys. 137, 194112 (2012)] rigorously defines the electronic energy on any subsystem within a molecule and gives a precise meaning to the subsystem ground and excited electronic energies, which is crucial for investigating electronic energy transfer from first principles. However, an efficient implementation of this approach has been hindered by complicated one- and two-electron integrals arising in its formulation. Using a resolution of the identity in the definition of partitioning we reformulate the method in a computationally e?cient manner that involves standard one- and two-electron integrals. We apply the developed algorithm to the 9-((1-naphthyl)-methyl)-anthracene (A1N) molecule by partitioning A1N into anthracenyl and CH2-naphthyl groups as subsystems, and examine their electronic energies and populations for several excited states using Configuration Interaction Singles method. The implemented approach shows a wide variety o...

Nagesh, Jayashree; Brumer, Paul

2014-01-01

405

Photoinduced energy and electron transfer in oligo(p-phenylene vinylene)-fullerene dyads  

Science.gov (United States)

The intramolecular photoinduced charge separation within an oligo(p-phenylene vinylene)-fulleropyrrolidine dyad with four phenyl rings (OPV4-C60) has been investigated with femtosecond pump-probe spectroscopy in solvents of different polarity and in the solid state. In solution, photoexcitation of the OPV4 moiety of OPV4-C60 results in an ultrafast (<190 fs) singlet energy transfer reaction, creating the fullerene singlet excited state. In polar solvents, the ultrafast energy transfer is followed in the picosecond time domain by an intramolecular electron transfer. In accordance with Marcus theory, the rates for forward and backward intramolecular electron transfer in OPV4-C60 are influenced by the polarity of the solvent. In the solid state the photophysics of OPV4-C60 is dramatically different. In thin films, the forward electron transfer proceeds within 500 fs, irrespective of which chromophore is photoexcited. The increased rate for charge separation in the solid state is attributed to a more favorable orientation of the donor and acceptor that results in an intermolecular electron transfer. In the films, energy and electron transfer processes compete at the earliest moments after photoexcitation. In the solid state, the photogenerated electrons and holes have long lifetimes as a result of migration of these charges to thermodynamically more favorable sites in the film.

van Hal, P. A.; Meskers, S. C. J.; Janssen, R. A. J.

406

Photoinduced energy and electron transfer in oligo(p-phenylene vinylene)-fullerene dyads  

Energy Technology Data Exchange (ETDEWEB)

The intramolecular photoinduced charge separation within an oligo(p-phenylene vinylene)-fulleropyrrolidine dyad with four phenyl rings (OPV4-C{sub 60}) has been investigated with femtosecond pump-probe spectroscopy in solvents of different polarity and in the solid state. In solution, photoexcitation of the OPV4 moiety of OPV4-C{sub 60} results in an ultrafast (<190 fs) singlet energy transfer reaction, creating the fullerene singlet excited state. In polar solvents, the ultrafast energy transfer is followed in the picosecond time domain by an intramolecular electron transfer. In accordance with Marcus theory, the rates for forward and backward intramolecular electron transfer in OPV4-C{sub 60} are influenced by the polarity of the solvent. In the solid state the photophysics of OPV4-C{sub 60} is dramatically different. In thin films, the forward electron transfer proceeds within 500 fs, irrespective of which chromophore is photoexcited. The increased rate for charge separation in the solid state is attributed to a more favorable orientation of the donor and acceptor that results in an intermolecular electron transfer. In the films, energy and electron transfer processes compete at the earliest moments after photoexcitation. In the solid state, the photogenerated electrons and holes have long lifetimes as a result of migration of these charges to thermodynamically more favorable sites in the film. (orig.)

Hal, P.A. van; Meskers, S.C.J.; Janssen, R.A.J. [Molecular Materials and Nanosystems, Laboratory for Macromolecular and Organic Chemistry, Eindhoven University of Technology, PO Box 513, 5600 MB, Eindhoven (Netherlands)

2004-06-01

407

Surface nanostructuring of LiNbO{sub 3} by high-density electronic excitations  

Energy Technology Data Exchange (ETDEWEB)

Lithium niobate (LiNbO{sub 3}) single crystals were irradiated with high energy gold ions (0.5–2.2 GeV) at the UNILAC (GSI) and with 150-keV highly charged xenon ions from an EBIT (Electron Beam Ion Trap, HZDR). The surfaces of the irradiated crystals were analyzed by scanning force microscopy showing very similar topographic changes. Swift heavy ions and slow highly charged ions produce hillock-like nanostructures on the surface. In both cases, the energy deposition of the ions is characterized by dense localized electronic excitations and efficient transfer to the lattice. Furthermore, the irradiation results in a shift in the band gap energy as evidenced by UV–Vis absorption spectroscopy. Specific modifications (e.g. hillock size, energy loss threshold) induced by slow highly charged ions are discussed in comparison with effects due to the electronic energy loss by swift heavy ions.

El-Said, A.S., E-mail: elsaid@kfupm.edu.sa [Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), 01328 Dresden (Germany); Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261 (Saudi Arabia); Nuclear and Radiation Physics Lab, Physics Department, Faculty of Science, Mansoura University, 35516 Mansoura (Egypt); Wilhelm, R.A. [Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), 01328 Dresden (Germany); Technische Universität Dresden, 01062 Dresden (Germany); Facsko, S. [Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), 01328 Dresden (Germany); Trautmann, C. [GSI Helmholtz Centre for Heavy Ion Research, 64291 Darmstadt (Germany); Technische Universität Darmstadt, 64289 Darmstadt (Germany)

2013-11-15

408

Surface nanostructuring of LiNbO3 by high-density electronic excitations  

International Nuclear Information System (INIS)

Lithium niobate (LiNbO3) single crystals were irradiated with high energy gold ions (0.5–2.2 GeV) at the UNILAC (GSI) and with 150-keV highly charged xenon ions from an EBIT (Electron Beam Ion Trap, HZDR). The surfaces of the irradiated crystals were analyzed by scanning force microscopy showing very similar topographic changes. Swift heavy ions and slow highly charged ions produce hillock-like nanostructures on the surface. In both cases, the energy deposition of the ions is characterized by dense localized electronic excitations and efficient transfer to the lattice. Furthermore, the irradiation results in a shift in the band gap energy as evidenced by UV–Vis absorption spectroscopy. Specific modifications (e.g. hillock size, energy loss threshold) induced by slow highly charged ions are discussed in comparison with effects due to the electronic energy loss by swift heavy ions

409

Electron scattering by magnesium: excitation of the 3s3p 1P1 state  

International Nuclear Information System (INIS)

Differential cross sections (DCS) for electron-impact excitation of the 3s3p 1P1 state in magnesium at incident electron energies E0 = 10, 15, 20, 40, 60, 80 and 100 eV have been measured and corresponding calculations carried out. Scattered-electron intensities were measured over a wide range of scattering angles (100-1500) and normalized to the DCSs at 100 experimentally obtained by Filipovic et al (2006 Int. J. Mass Spectrom. 251 66). Corresponding calculations have been conducted in the relativistic distorted-wave approximation. Integrated (integral, momentum transfer and viscosity) cross sections are determined by numerical integration of our DCSs. The results are analysed and compared with previous experimental data and theoretical calculations

410

Electron transfer in gas surface collisions  

International Nuclear Information System (INIS)

In this thesis electron transfer between atoms and metal surfaces in general is discussed and the negative ionization of hydrogen by scattering protons at a cesiated crystalline tungsten (110) surface in particular. Experimental results and a novel theoretical analysis are presented. In Chapter I a theoretical overview of resonant electron transitions between atoms and metals is given. In the first part of chapter II atom-metal electron transitions at a fixed atom-metal distance are described on the basis of a model developed by Gadzuk. In the second part the influence of the motion of the atom on the atomic charge state is incorporated. Measurements presented in chapter III show a strong dependence of the fraction of negatively charged H atoms scattered at cesiated tungsten, on the normal as well as the parallel velocity component. In chapter IV the proposed mechanism for the parallel velocity effect is incorporated in the amplitude method. The scattering process of protons incident under grazing angles on a cesium covered surface is studied in chapter V. (Auth.)

411

Charge transfer: An electronic bath approach  

International Nuclear Information System (INIS)

A charge transfer theory is developed for systems in which an atom, molecule, or ion interacts for a period of time with a substrate. The Hamiltonian of the system is taken to be one-electron and the nuclear motion of the substrate is ignored. The theory is structured so as to mimic generalized Langevin theory; namely, the equations of motion for annihilation operators of spin orbitals localized at the interaction site are developed such that a primary zone is defined with the remainder of the substrate being included through a memory kernel and a driving term. An electronic bath approximation is presented which leads to equations of motion for the occupation numbers of spin orbitals in the primary zone. These equations have the property that primary-zone occupation numbers damp to an equilibrium state at long time independent of their initial values. Specific application to a substrate modeled as a semi-infinite, Huckel chain is presented and numerical results are obtained. The electronic bath approximation is found to produce results in agreement with numerically exact data

412

Relaxation of electron excitations in beryllium oxide. Auto localized excitons  

International Nuclear Information System (INIS)

Combined luminescent-optical investigations within 80-600 K temperature range and 5-36 eV energy range is conducted for BeO nominally pure crystals. Analysis of selective excitation spectra of luminescence, kinetic parameters and temperature dependences of the selected luminosity, as well as, of spectra of transit optical absorption induced by electron beam, is presented. Models of auto localized excitons differing in configuration of hole component are suggested and fact of coexistence of high and low radii excitons is determined on the basis of the analysis with regard to peculiarities of BeO crystalline structure. 40 refs.; 8 figs

413

Ion-acoustic envelope excitations in electron-positron-ion plasma with nonthermal electrons  

Energy Technology Data Exchange (ETDEWEB)

A theoretical investigation has been made for modulational instability of ion-acoustic waves in an electron-positron-ion plasma with nonthermal electrons. Employing reductive perturbation method (RPM), the nonlinear Schroedinger equation (NLSE) is derived. The dispersive and nonlinearity coefficients, P and Q, respectively, are the functions of nonthermal parameter ({beta}), ratio of positron to electron density (d), ratio of the electron to positron temperature ({delta}) and ratio of ion to electron temperature ({sigma}). It is observed that these parameters significantly modify the conditions of the modulational instability. The system supports both types of bright and dark envelope excitations.

Gill, Tarsem Singh, E-mail: gillsema@yahoo.co.i [Department of Physics, Guru Nanak Dev University, Amritsar 143005 (India); Bains, Amandeep Singh [Department of Physics, Guru Nanak Dev University, Amritsar 143005 (India); Saini, Nareshpal Singh, E-mail: nssaini@yahoo.co [Department of Physics, Guru Nanak Dev University, Amritsar 143005 (India); Center for Plasma Physics, Department of Physics and Astronomy, Queen' s University Belfast, BT7 1NN (United Kingdom); Bedi, Chanchal [Department of Physics, Guru Nanak Dev University, Amritsar 143005 (India)

2010-07-12

414

Ion-acoustic envelope excitations in electron-positron-ion plasma with nonthermal electrons  

International Nuclear Information System (INIS)

A theoretical investigation has been made for modulational instability of ion-acoustic waves in an electron-positron-ion plasma with nonthermal electrons. Employing reductive perturbation method (RPM), the nonlinear Schroedinger equation (NLSE) is derived. The dispersive and nonlinearity coefficients, P and Q, respectively, are the functions of nonthermal parameter (?), ratio of positron to electron density (d), ratio of the electron to positron temperature (?) and ratio of ion to electron temperature (?). It is observed that these parameters significantly modify the conditions of the modulational instability. The system supports both types of bright and dark envelope excitations.

415

Elastic scattering and vibrational excitation cross sections for electron collisions with C2F6  

International Nuclear Information System (INIS)

Absolute elastic cross sections for e-C2F6 collisions have been measured for impact energies of 2 to 100 eV and scattering angles from 10 to 130oC. From these, the integrated and momentum transfer cross sections were obtained by extrapolation and numerical integration. Energy loss spectra were measured for vibrational excitation. The vibrational excitation functions show resonances at 4.3 eV and 8.5 eV. By decomposition of the loss spectra and symmetry analysis according to angular correlation theory, the resonances have been assigned to a temporary trapping of the electrons in the a2u (4.3 eV) and eu (8.5 eV) antibonding orbitals of the molecule. (author)

416

New Excited-State Proton Transfer Mechanisms for 1,8-Dihydroxydibenzo[a,h]phenazine.  

Science.gov (United States)

The excited state intramolecular proton transfer (ESIPT) mechanisms of 1,8-dihydroxydibenzo[a,h]phenazine (DHBP) in toluene solvent have been investigated based on time-dependent density functional theory (TD-DFT). The results suggest that both a single and double proton transfer mechanisms are relevant, in constrast to the prediction of a single one proposed previously (Piechowska et al. J. Phys. Chem. A 2014, 118, 144-151). The calculated results show that the intramolecular hydrogen bonds were formed in the S0 state, and upon excitation, the intramolecular hydrogen bonds between -OH group and pyridine-type nitrogen atom would be strengthened in the S1 state, which can facilitate the proton transfer process effectively. The calculated vertical excitation energies in the S0 and S1 states reproduce the experimental UV-vis absorption and fluorescence spectra well. The constructed potential energy surfaces of the S0 and S1 states have been used to explain the proton transfer process. Four minima have been found on the S1 state surface, with potential barriers between these excited-state minima of less than 10 kcal/mol, which supports concomitant single and double proton transfer mechanisms. In addition, the fluorescence quenching can be explained reasonably based on the proton transfer process. PMID:25555144

Zhao, Jinfeng; Yao, Hongbin; Liu, Jianyong; Hoffmann, Mark R

2015-01-29

417

Ultrafast competition between energy and charge transfer in a functionalized electron donor/fullerene derivative  

Science.gov (United States)

The fact that fullerenes are good electron acceptors has generated interest in covalently linked complexes between electron donors and fullerenes; photoinduced charge transfer in these dyads has great potential for use in photovoltaic devices. In this Letter, we investigate the excited-state properties of a perylene-fulleropyrrolidine dyad using steady-state and femtosecond time-resolved spectroscopies. Following photoexcitation, charge separation and energy transfer occur in nearly equal proportion; both processes take place on a sub-picosecond timescale. This suggests that competition between energy and charge transfer is common in these molecular systems, so that the best molecules for device applications are not necessarily those with the fastest electron transfer rates.

Martini, Ignacio B.; Ma, Bin; Da Ros, Tatiana; Helgeson, Roger; Wudl, Fred; Schwartz, Benjamin J.

2000-09-01

418

New evidence for resonant-transfer-and-excitation for 16S, 20Ca, and 23V ions colliding with helium  

International Nuclear Information System (INIS)

Significant new evidence is presented for resonant-transfer-and-excitation (RTE) in ion-atom collisions. This process occurs when a target electron is captured simultaneously with excitation of the projectile followed by deexcitation via photon emission. RTE, which is analogous to dielectronic recombination (DR), proceeds via an inverse Auger transition, and is expected to be resonant for projectile velocities corresponding to the energy of the ejected electron in the Auger process. Cross sections for projectile K x-ray emission coincident with single electron capture have been measured for 70 to 160 MeV 16S13+, 100 to 360 MeV 20Ca16+17+18+ and 180 to 460 MeV 23V19+20+21+ ions colliding with helium. Strong resonant behavior, in agreement with theoretical calculations of RTE, is observed as a function of beam energy in the measured cross sections. For Ca and V ions two maxima are observed in the energy dependence of the measured coincidence cross sections. Theoretical calculations show that these maxima are correlated to two groups of intermediate resonant states in the RTE process for which the excited and captured electrons occupy levels with principal quantum numbers n = 2,2 and n = 2, greater than or equal to 3. 13 references, 6 figures