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.

Anthryldiene derivatives carrying electron withdrawing end group displayed wavelength dependant regio selective E(trans) ? Z(cis) isomerization from the singlet excited state. Fluorescence studies indicated the highly polarized/charge-transfer nature of the singlet excited state.   

The mechanism of deactivation of energy of excitation in a resin system was investigated on optical excitation as well as excitation by high energy electrons. This mechanism involves formation of excited state complexes, known as exciplexes which have a considerable charge transfer character. This mechanism will be used to develop a degradation model for epoxy matrix materials deployed in a space environment.

This research project involves the design, synthesis and study of 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).

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).

Electron transfer reactions have been extensively studied for atoms in ground state colliding with molecules. On the contrary, few experiments have been performed for atoms excited above the first resonance states and it has been supposed by some authors that above a given degree of excitation, electron transfer becomes negligeable. By using a simple Landau-Zener model and a LCAO calculation of the coupling matrix element between ionic and covalent states for excited atoms and molecules with small electron affinities, we conclude that there is no limit to the electron transfer, in agreement with the experimental observation of ion pair formation for Rydberg atoms colliding with given molecules such as SF/sub 6/.

With the two dimensional (2D) site and the three dimensional (3D) cube representations in two-photon absorption [M.T. Sun, J.N. Chen, H.X. Xu, J. Chem. Phys. 128(1-6) (2008) 064106], the charge transfer and electron-hole coherence for centrosymmetric and asymmetric fluorene derivatives have been investigated theoretically. For centrosymmetric fluorene derivative, the excitation from the ground state to intermediate excited state by the first photon is a Frenkel excitation; while the excitation by the second photon from the intermediate excited state to the final excited state is an intraband excitation, and charge transfers from one substituent to the other substituent. For asymmetric fluorene derivative in TPA, the excitations by the first and second photons are all charge transfer excite...

This research project involves the design, synthesis and study of molecules which mimic many of the important aspects of photosynthetic electron and energy transfer. The knowledge gained from the study of synthetic model systems which abstract features of the natural photosynthetic apparatus can be used to design artificial photosynthetic systems which employ the basic physics and chemistry of photosynthesis to help meet mankind`s energy needs. More specifically, the proposed models 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.

The kinetics of excitation energy transfer and electron transfer processes within the membrane of Heliobacillus mobilis were investigated using femtosecond transient absorption difference spectroscopy at room temperature. The kinetics in the 725- to 865-nm region, upon excitation at 590 and 670 nm, ...

Single molecule fluorescence spectroscopy was used to study photoinduced electron transfer (ET) dynamics across single donor-bridge-acceptor junctions consisting of perylene-3,4:9,10-bis(dicarboximide) (PDI), n-phenylene bridge with COOH anchoring group, and antimony doped Tin Oxide(ATO) nanoparticles. Photo-excitation of PDI initiates electron transfer from its excited state into ATO nanoparticles. Electron transfer was confirmed and ensemble average rate was measured by transient infrared absorption spectroscopy, in which injected electrons in ATO were directly monitored. Single molecule fluorescence from donor molecule was confirmed by the observed blinking behavior, fluorescence spectrum, and excitation polarization dependence. Single molecule fluorescence lifetime was measured by time-correlated single photon counting, from which forward electron transfer rate from adsorbate excited state to nanoparticle was determined. The dependence of these single molecule ET rates and their fluctuation on the length of phenylene bridge and the nature of semiconductors are being investigated.

In the photosensitized electron transfer reaction of 6,6-diphenyl-1,4-dioxaspiro[4.5]decane in methanol (MeOH), the quantum yield is increased by the use of 2-methylbenzene-1,4-dicarbonitrile (2-methyl-BDC) or 2,5-dimethylbenzene-1,4-dicarbonitrile (2,5-dimethyl-BDC) as compared to that by the use of 1,4-benzenedicarbonitrile (BDC), though the excited methyl-substituted BDCs have lower reduction potentials than the excited BDC. Detailed study on this substituent effect reveals that the electron transfer between MeOH and the excited BDC occurs to some extent. By the use of methylated BDCs, electron transfer between the excited sensitizers and MeOH is suppressed enough due to its lower reduction potential. Some typical photoinduced electron transfer reactions in alcohol proceed more efficiently by the use of 2-methyl-BDC or 2,5-dimethyl-BDC instead of BDC.   

Double resonant polarization labeling spectroscopy is applied to detect nitric oxide in flames and to characterize rotational energy transfer and orientation changing collisions in its first excited electronic state. (author) 4 figs., 3 refs.

By combining picosecond optical experiments and detailed statistical mechanics theory we continue to increase our understanding of the complex interplay of structure and dynamics in important energy transfer situations. A number of different types of problems will be focused on experimentally and theoretically. They are excitation transport among chromophores attached to finite size polymer coils; excitation transport among chromophores in monolayers, bilayers, and finite and infinite stacks of layers; excitation transport in large vesicle systems; and photoinduced electron transfer in glasses and liquids, focusing particularly on the back transfer of the electron from the photogenerated radical anion to the radical cation. 33 refs., 13 figs.

The simultaneous, concerted transfer of electrons and protons—electron-proton transfer (EPT)—is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following ph...

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.

Resonant electron transfer and double excitation (RME) is a correlated electron process which is expected to occur in an ion-atom collision when electron capture is accompanied by the simultaneous excitation of two inner-shell electrons. RT2 is similar to resonant transfer excitation (RTE) in which only a single electron is excited. RT2E was investigated experimentally for 38--42 MeV/u Kr{sup 34} + H{sub 2} collisions by observing x-ray emission associated with single-electron capture. No events associated with Kr K x rays (near 13 keV were observed; however, events do occur at about twice (> 22 keV) the Kr K x-ray energy. Several possible sources of these latter x rays have been considered.

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)

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)

Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated. Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtosecond time-resolved vibrational spectroscopy is used to directly monitor the ultrafast dynamical behavior of hydrogen bonds in the electronic excited state. It is important to note that the excited-state hydrogen-bonding dynamics are coupled to the electronic excitation. Fortunately, the combination of femtosecond time-resolved spectroscopy and accurate quantum chemistry calculations of excited states resolves this issue in laser experiments. Through a comparison of the hydrogen-bonded complex to the separated hydrogen donor or acceptor in ground and electronic excited states, the excited-state hydrogen-bonding structure and dynamics have been obtained. Moreover, we have also demonstrated the importance of hydrogen bonding in many photophysical processes and photochemical reactions. In this Account, we review our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of electronic excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that the intermolecular hydrogen bond in the electronic excited state is greatly strengthened for coumarin chromophores and weakened for thiocarbonyl chromophores. We have also clarified that the intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced through the regulation of electronic states by hydrogen-bonding interactions. Consequently, the fluorescence of chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Our research expands our understanding of the nature of hydrogen bonding by delineating the interaction between hydrogen bonds and photons, thereby providing a basis for excited-state hydrogen bonding studies in photophysics, photochemistry, and photobiology. PMID:22070387

A theory for simultaneous charge and energy transfer in the carotenoid-chlorophyll-a complex is presented here and discussed. The observed charge transfer process in these chloroplast complexes is reasonably explained in terms of this theory. In addition, the process leads to a mechanism to drive an electron in a lower to a higher-energy state, thus providing a mechanism for the ejection of the electron to a nearby molecule (chlorophyll) or into the environment. The observed lifetimes of the electronically excited states are in accord/agreement with the investigations of Sundström et al. and are in the range of pico-seconds and less. The change in electronic charge distribution in internuclear space as the system undergoes an electronic transition to a higher-energy state could, under appropriate physical conditions, lead to oscillating dipoles capable of transmitting energy from the carotenoid-chlorophylls chromophore to the reaction center by sending an electromagnetic wave (a photon) which provides a novelnew mechanism for energy production. In the simplest version of the Förster–Dexter theory, the excitation energy of a donor is transferred to an acceptor and then de-excited to the ground state by fluorescence with no electron being transferred. In the process proposed herein, charge and energy both are transferred from donor to acceptor which can further de-excite by fluorescence. The charge transfer time scale involving an actual transfer of electron is in the pico-second range.

Photochemical polymerizations of dithienothiophenes were investigated. Photo-irradiation to the solution containing dithienothiophene and appropriate electron acceptor gave poly-dithienothiophene having ca. 104 (vs. PSt.) of molecular weight. The cation radical of the monomer was efficiently generated through photoinduced electron transfer from the excited state of the monomer to the electron acceptor. Then the successive coupling reaction resulted in a formation of the polymers.   

We investigate the control of electronic energy transfer in molecular dimers through the preparation of specific vibrational coherences prior to electronic excitation, and its observation by nonlinear wave-packet interferometry. Laser-driven coherent nuclear motion can affect the instantaneous resonance between site-excited electronic states and thereby influence short-time electronic excitation transfer (EET). We first illustrate this control mechanism with calculations on a dimer whose constituent monomers undergo harmonic vibrations. We then consider the use of nonlinear wave-packet interferometry (nl-WPI) experiments to monitor the nuclear dynamics accompanying EET in general dimer complexes following impulsive vibrational excitation by a sub-resonant control pulse (or control pulse sequence). In measurements of this kind, two pairs of polarized phase-related femtosecond pulses following the control pulse generate superpositions of coherent nuclear wave packets in optically accessible electronic states. I...

Phenol-ammonia clusters with more than five ammonia molecules are proton transferred species in the ground state. In the present work, the excited states of these zwitterionic clusters have been studied experimentally with two-color pump probe methods on the nanosecond time scale and by ab initio electronic-structure calculations. The experiments reveal the existence of a long-lived excited electronic state with a lifetime in the 50-100 ns range, much longer than the excited state lifetime of bare phenol and small clusters of phenol with ammonia. The ab initio calculations indicate that this long-lived excited state corresponds to a biradicalic system, consisting of a phenoxy radical that is hydrogen bonded to a hydrogenated ammonia cluster. The biradical is formed from the locally excited state of the phenolate anion via an electron transfer process, which neutralizes the charge separation of the ground state zwitterion.

Tunneling electrons in a scanning tunneling microscope were used to excite specific vibrational quantum states of adsorbed water and hydroxyl molecules on a Ru(0 0 0 1) surface. The excited molecules relaxed by transfer of energy to lower energy modes, resulting in diffusion, dissociation, desorption, and surface-tip transfer processes. Diffusion of H{sub 2}O molecules could be induced by excitation of the O-H stretch vibration mode at 445 meV. Isolated molecules required excitation of one single quantum while molecules bonded to a C atom required at least two quanta. Dissociation of single H{sub 2}O molecules into H and OH required electron energies of 1 eV or higher while dissociation of OH required at least 2 eV electrons. In contrast, water molecules forming part of a cluster could be dissociated with electron energies of 0.5 eV.

Two-center semiclassical close-coupling method is applied to study collisions of He atom by bare and multiply charged ions (Be{sup q+} (q=2-4) and B{sup q+} (q=2-5)) in the energy range of 10-500 keV/amu. Cross sections for one-electron processes for electron capture, excitation, ionization as well as two-electron processes for transfer excitation and transfer ionization were obtained. Dependence of cross section on projectile energy and charge state was investigated. These calculations provided the first systematic study for these collision systems. Comparison with the limited number of previous theoretical and experimental results was also made.

This project involved the experimental probing of the electronic excited states generated by photoinduced (center-to-center) electron and energy transfer processes in several classes of transition metal donor/acceptor (D/A) complexes. Some of the general properties inferred from these studies should be useful in the design of new systems for energy conversion applications. Pursuit of the project goals has involved the determination of electron transfer efficiencies and the detailed study of variations in the electronic spectra of D/A complexes. This has resulted in the study of some very fundamental issues of photoinduced charge transfer and the identification of some of the constraints on its efficiency. The experimental studies of the competition between the degradative non-radiative unimolecular relaxation of transition metal excited states and their transfer of charge from these excited states to external acceptors have involved a range of techniques such as transient decay kinetics, photoacoustic calorimetry and transient or stationary state spectroscopy. The substrates synthesized for these studies were selected to provide model systems, or series of model systems to probe the validity of models of electronic excited states and their reactivity. The work during the last few years has focused largely, but not exclusively, on the use of emission spectral band shapes to probe the properties of charge transfer (CT) excited states. Bandshape variations are one of the very few approaches for systematically probing electronic excited states and good band shape resolution is necessary in order to gain information about the structural variations that correlate with excited state reactivity. Differences in molecular structure correlate with differences in chemical reactivity, and the variations in emission bandshapes are well known to relate to variations in the molecular structural differences between the excited and ground electronic states. However, it is has been rarely noticed that configurational mixing of the lowest energy excited state with other electronic states leads to unique distortions of the lowest energy excited state which result in modifications in the vibronic structure and bandshape of the emission. We have used the emission sideband shapes to evaluate patterns of ground state-excited state and excited state-excited state configurational mixing in some simple series of complexes.

Fullerene-bridge-aniline dyad and the model fulleropyrrolidine compound form stable, optically transparent clusters in mixtures (3:1) of acetonitrile and toluene. Ground- and excited-state properties of the clusters of the dyad and the model compound are compared with their corresponding monomeric forms. Clustering of the dyad as well as the model compound exhibits a red-shifted emission maximum ({lambda}{sub max} {approximately}738 nm) compared to their monomeric forms ({lambda}{sub max} {approximately}714 nm). The electron transfer from the appended electron donor moiety to the parent fullerene core in the dyad cluster is evident from the decreased ({approximately}80%) fluorescence yield. The formation of fullerene radical anion (absorption maximum at 1010nm) with a lifetime of several hundreds of microseconds was further confirmed using nanosecond laser (337 nm) flash photolysis experiments. In contrast, the dyad molecules in their monomeric form did not yield any detectable yield of C{sub 60} radical anion following laser pulse excitation. The failure to observe any charge-transfer intermediates following laser pulse excitation. The failure to observe any charge-transfer intermediates following laser pulse excitation, even in polar solvents such as benzonitrile or nitromethane, suggested that fast back-electron-transfer process must be operative in the monomeric dyad system. On the other hand, clustering of the fullerene-based dyads in a mixed-solvent system can provide a unique way to decrease the rate of back electron transfer, thus stabilizing the electron-transfer products.

Photoinduced electron transfer in photosystem I (PS I) proceeds from the excited primary electron donor P700 (a chlorophyll a dimer) via the primary acceptor A0 (chlorophyll a) and the secondary acceptor A1 (phylloquinone) to three [4Fe-4S] clusters, Fx, FA, and FB. Prereduction of the iron-sulfur c...

Finite cluster models and a variety of ab initio wave functions have been used to study the electronic structure of bulk KNiF3. Several electronic states, including the ground state and some charge-transfer excited states, have been considered. The study of the cluster-model wave functions has permi...

Momentum-resolved resonant inelastic x-ray scattering (RIXS) spectroscopy has been carried out successfully at the Fe K-edge for the first time. The RIXS spectra of a FeBO3 single crystal reveal a wealth of information on ? 1-10 eV electronic excitations. The IXS signal resonates when the incident photon energy approaches the pre-edge (1s¯-3d) and the main-edge (1s¯-4p) of the Fe K-edge absorption spectrum. The RIXS spectra measured at the pre-edge and the main-edge show quantitatively different dependences on the incident photon energy, momentum transfer, photon polarization, and temperature. We present a multielectron analysis of the Mott-Hubbard (MH) and charge transfer (CT) excitations, and calculate their energies. Electronic excitations observed in the pre-edge and main-edge RIXS spectra are interpreted as MH and CT excitations, respectively. We propose the electronic structure around the chemical potential in FeBO3 based on the experimental data.

The calculation of the electron density and electron temperature distribution in our ionosphere (from {approx} 150-600 km) requires a knowledge of the various heating, cooling and energy flow processes that occur. The energy transfer from electrons to neutral gases and ions is one of the dominant electron cooling processes in the ionosphere, and the role of vibrationally excited N2 in this is particularly significant.

This volume contains the abstracts of 29 formal presentations and 32 posters, the program of the meeting, and a list of attendees. The scope of papers presented includes: photoelectrolysis, photodissociation of water, photochemical energy storage, metalloporphyrin electrode films, photoelectrochemistry of new phenothiazine, photoinduced electron transfer, biomimetic solar energy conversion systems, intermediates from transition metal excited-state electron transfer reactions, photosynthetic hydrogen production, photophysics of chlorophyll, and studies of photosynthetic reaction centers. (DMC)

Electron transfer (ET) processes in proteins are characterized by the motion of a single electron between centers of localization (such as the chlorophyll dimer in photosynthetic reaction centers). An electronic donor state D is created by the injection of an electron or by photo-excitation, after which the system makes a radiationless transition to an acceptor state A., resulting in the effective transfer of an electron over several angstroms. The experimental and theoretical understanding of the rate of this process has been the focus of much attention in physics, chemistry and biology.

Two rhenium(I) tricarbonyl diimine complexes, one of them with a 2,2'-bipyrazine (bpz) and a pyridine (py) ligand in addition to the carbonyls ([Re(bpz)(CO)(3)(py)](+)), and one tricarbonyl complex with a 2,2'-bipyridine (bpy) and a 1,4-pyrazine (pz) ligand ([Re(bpy)(CO)(3)(pz)](+)) were synthesized, and their photochemistry with 4-cyanophenol in acetonitrile solution was explored. Metal-to-ligand charge transfer (MLCT) excitation occurs toward the protonatable bpz ligand in the [Re(bpz)(CO)(3)(py)](+) complex while in the [Re(bpy)(CO)(3)(pz)](+) complex the same type of excitation promotes an electron away from the protonatable pz ligand. This study aimed to explore how this difference in electronic excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled electron transfer (PCET) between 4-cyanophenol and the two rhenium(I) complexes. Transient absorption spectroscopy provides clear evidence for PCET reaction products, and significant H/D kinetic isotope effects are observed in some of the luminescence quenching experiments. Concerted proton-electron transfer is likely to play an important role in both cases, but a reaction sequence of proton transfer and electron transfer steps cannot be fully excluded for the 4-cyanophenol/[Re(bpz)(CO)(3)(py)](+) reaction couple. Interestingly, the rate constants for bimolecular excited-state quenching are on the same order of magnitude for both rhenium(I) complexes. PMID:22804105

We report resonant inelastic x-ray scattering (RIXS) measurements on polycrystalline and single crystal samples of the organic semiconductor {beta}-copper phthalocyanine (CuPc) as well as time dependent density functional theory calculations of the electronic properties of the CuPc molecule. Resonant and nonresonant excitations were measured along the three crystal axes with 120 meV resolution. We observe molecular excitations as well as charge-transfer excitons along certain crystal directions and compare our data with the calculations. Our results demonstrate that RIXS is a powerful tool for studying excitons and other electronic excitations in organic semiconductors.

We report resonant inelastic x-ray scattering studies of electronic excitations in a wide variety of cuprate compounds. Specifically, we focus on the charge-transfer type excitation of an electron from a bonding molecular orbital to an antibonding molecular orbital in a copper oxygen plaquette. Both the excitation energy and the amount of dispersion are found to increase significantly as the copper oxygen bond-length is reduced. We also find that the estimated bond-length dependence of the hopping integral t_pd is much stronger than that expected from tight-binding theory.

An electron donor-acceptor dyad (quaterthiophene-anthraquinone) mediates ultrafast intramolecular photoinduced charge separation and consequent charge recombination when in polar or moderately polar solvents. Alternatively, non-polar media completely impedes the initial photoinduced electron transfer by causing enough destabilization of the charge-transfer state and shifting its energy above the energy of the lowest locally excited singlet state. Furthermore, femtosecond transient-absorption spectroscopy reveals that for the solvents mediating the initial photoinduced electron-transfer process, the charge recombination rates were slower than the rates of charge separation. This behavior of donor-acceptor systems is essential for solar-energy-conversion applications. For the donor-acceptor ...

Porphyrin and fullerene donor-acceptor complexes have been extensively studied for their photo-induced charge transfer characteristics. We present the electronic structure of ground states and a few charge transfer excited states of four cofacial porphyrin-fullerene molecular constructs studied using density functional theory at the all-electron level using large polarized basis sets. The donors are base and Zn-tetraphenyl porphyrins and the acceptor molecules are C60 and C70. The complexes reported here are non-bonded with a face-to-face distance between the porphyrin and the fullerene of 2.7 to 3.0 A?. The energies of the low lying excited states including charge transfer states calculated using our recent excited state method are in good agreement with available experimental values. We find that replacing C60 by C70 in a given dyad may increase the lowest charge transfer excitation energy by about 0.27 eV. Variation of donor in these complexes has marginal effect on the lowest charge transfer excitation energy. The interfacial dipole moments and lowest charge transfer states are studied as a function of face-to-face distance.

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.

In an ion-atom collision projectile excitation and charge transfer (electron capture) may occur together in a single encounter. If the excitation and capture are correlated, then the process is called resonant transfer and excitation (RTE); if they are uncorrelated, then the process is termed nonresonant transfer and excitation (NTE). Experimental work to date has shown the existence of RTE and provided strong evidence for NTE. Results presented here provide information on the relative magnitudes of RTE and NTE, the charge state dependence of RTE, the effect of the target momentum distribution on RTE, the magnitude of L-shell RTE compared to K-shell RTE, and the target Z dependences of RTE and NTE. 15 refs., 5 figs.

The effective absorption cross-section of a molecule (acceptor) can be greatly increased by associating it with a cluster of molecules that absorb light and transfer the excitation energy to the acceptor molecule. The basic mechanism of such light harvesting by Förster resonance energy transfer (FRET) is well established, but recent experiments have revealed a new feature whereby excitation is coherently shared among donor and acceptor molecules during FRET. In the present study, two-dimensional electronic spectroscopy was used to examine energy transfer at ambient temperature in a naturally occurring light-harvesting protein (PE545 of the marine cryptophyte alga Rhodomonas sp. strain CS24). Quantum beating was observed across a range of excitation frequencies. The shapes of those features in the two-dimensional spectra were examined. Through simulations, we show that two-dimensional electronic spectroscopy provides a probe of the adiabaticity of the free energy landscape underlying light harvesting.

Tris[2-{3-(dimesitylboryl)phenyl}pyridinato]iridium(III) ([Ir(Bppy)3]) showed bright green-phosphorescence with the emission lifetime and quantum yield of 1.0–1.4 µs and 1.0, respectively, in nonpolar–polar solvents at 298 K. Detailed spectroscopic experiments and DFT calculations demonstrated that the emissive state of the complex was the metal-to-boron charge-transfer (MBCT) excited state, in which the vacant p-orbital on the boron atom was occupied by the excited electron.   

We report on the calculated elastic differential, elastic integral, momentum transfer, and excitation cross sections for the low-energy electron-NH{sub 3} scattering using the R-matrix method. Elastic differential and momentum transfer cross sections are obtained by summing over rotationally elastic and rotationally inelastic cross sections for rotor states up to J=4. The excitation cross sections of the first four low-lying electronically excited states from the ground state of NH{sub 3}, at its equilibrium geometry, are presented. These excited states have symmetries a {sup 3}A{sub 1}, A {sup 1}A{sub 1}, b {sup 3}E, and B {sup 1}E. The set of self-consistent-field molecular orbitals is obtained by optimizing these on the first excited state {sup 3}A{sub 1}. Configuration-interaction (CI) wave functions are used to represent the target states. In our CI model, we kept the core two electrons frozen in doubly occupied molecular orbital 1a{sub 1}, and the remaining eight electrons moved freely among the five molecular orbitals 2a{sub 1}, 3a{sub 1}, 1e, 4a{sub 1}, 2e. With this CI model, we obtain good agreement for the vertical spectrum of excited states with the experimental values. The Born approximation is employed to account for higher partial waves excluded in the R-matrix method to evaluate elastic cross sections. Cross sections are reported for the electron-impact energy range 0.025-20 eV.

This project involves the design, synthesis and study of molecules which mimic some of the important aspects of photosynthetic electron and energy transfer. This research project is leading to a better understanding of the energy conserving steps of photosynthesis via the study of synthetic model systems which abstract features of the natural photosynthetic apparatus. The knowledge gained from these studies will aid in the design of artificial photosynthetic reaction centers which employ the basic chemistry and physics of photosynthesis to help meet mankind`s energy needs. The approach to artificial photosynthesis employed in this project is to use synthetic pigments, electron donors, and electron acceptors similar to those found in biological reaction centers, but to replace the protein component with covalent bonds. These chemical linkages determine the electronic coupling between the various moieties by controlling separation, relative orientation, and overlap of electronic orbitals. The model systems are designed to mimic the following aspects of natural photosynthetic electron transfer: electron donation from a tetrapyrrole excited single state, electron transfer between tetrapyrroles, electron transfer from tetrapyrroles to quinones, and electron transfer between quinones with different redox properties. In addition, they mimic 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).

(6-4) photoproduct, the second major DNA lesion induced by UV irradiation, is repaired by (6-4) photolyase using light energy. The molecular mechanism of enzymatic repair is poorly understood. Here we report the direct observation of catalytic processes by synchronizing the enzymatic dynamics with the repair function through femtosecond spectroscopy. We observed forward electron transfer from the excited flavin cofactor to damaged DNA at 225 ps, backward electron transfer from unrepaired DNA to flavin at 50 ps, and electron returns from repaired DNA to flavin at tens of nanoseconds. Strikingly, a 425-ps electron-induced proton transfer was observed for the first time, which is crucial for repair efficiency by competing with the non-repair backward electron transfer channel.

Charge-transfer-to-solvent (CTTS) reactions of hydroxide induced by 200 nm monophotonic or 337 nm and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, eaq-, have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component (ca. 13 ps) and a slower power "tail" due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% vs. 17.5% at 25 oC, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is < 4 cm TW-1 M-1 (assuming unity quantum efficiency for the photodetachment). For [O...

Electron-rich uranium coordination complexes display a pronounced reactivity toward small molecules. In this Feature article, the exciting chemistry of trivalent uranium ions coordinated to classic Werner-type ligand environments is reviewed. Three fundamentally important reactions of the [(((R)ArO)3tacn)U]-system are presented that result in alkane coordination, CO/CO2 activation, and nitrogen atom-transfer chemistry. PMID:16550268

A key step in the photosynthetic reactions in photosystem II of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photo...

In the photochemical cycle of the labelled, tris (2,2'bipyridine) ruthenium(II), Ru(bpy)32+, the photo excited label is oxidized through electron transfer quenching by methylviologen (l,l'-dimethyl-4,4'-bipyridinium dication, MV ) producing the Ru(III) complex and MV+*. The resulting radical cati...

In the presence of Lu3+ and Sc3+ ions, p-pentamethyldisilanylacetophenone shows dual fluorescence, characteristic of a phenylpentamethyldisilane with an electron-accepting substituent on benzene ring, where the wavelengths of the LE (locally excited) and ICT (intramolecular charge transfer) band maxima and their relative intensity are dependent on the metal ions.   

1-[(Anthracen-9-yl)methylene] thiosemicarbazide shows weak fluorescence due to a photo-induced electron transfer (PET) process from the thiosemicarbazide moiety to the excited anthracene. The anthracene emission can be recovered via protonation of the amine as the protonated aminomethylene as an ele...

In natural photosynthesis membranes, chlorophyll molecules serve as the site of the initial photodriven charge separation. In addition, they play a role in subsequent electron-transfer steps, accept singlet excitation energy from carotenoid antenna molecules, and transfer triplet energy to carotenoid acceptors (thereby preventing sensitized singlet oxygen production and subsequent photodamage to the organism). The authors report herein the synthesis and study of chlorophyll-based carotenopyropheophorbide-quinone triad molecules which mimic all of these natural processes.

2-(N,N-Dimethylamino)-5, 14-ethanopentacene (1) was synthesized and spectroscopic behaviors investigated. Results suggest that 1 undergoes photoinduced electron transfer (PET) in solvents more polar than saturated hydrocarbons. The resulting charge-transfer (CT) state undergoes CT fluorescence efficiently in solvents of low dielectric constants. Fluorescence excitation studies of the CT emission reveal the existence of an EDA interaction in the ground state. The implications of these results are discussed.

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+}.

Inelastic electron scattering off 110Cd and 104,106,108,110Pd was studied in an effective momentum transfer range 0.3-2.55 fm-1. The form-factor data were analyzed in a model-independent Fourier-Bessel expansion. Excitation energy spectra and transition strengths have been calculated in the framework of the interacting boson approximation, in a model space extended with one g-boson. The behaviour of the experimental transition charge densities extracted for hexadecapole excitations was used to study in detail effects of the addition of one hexadecapole g-boson on the dynamic properties of low-lying collective excitations in this mass region.

By means of time-resolved electron crystallography, we report direct observation of the structural dynamics of graphite, providing new insights into the processes involving coherent lattice motions and ultrafast graphene ablation. When graphite is excited by an ultrashort laser pulse, the excited carriers reach their equilibrium in less then one picosecond by transferring heat to a subset of strongly coupled optical phonons. The time-resolved diffraction data show that on such a time scale the crystal undergoes a contraction whose velocity depends on the excitation fluence. The contraction is followed by a large expansion which, at sufficiently high fluence, leads to the ablation of entire graphene layers, as recently predicted theoretically.

The photophysical properties of 4-(2-dimethylaminoethyloxy)-N-octadecyl-1,8-naphthalimide (DON) consisting of donor and acceptor units were investigated in different solutions. Changing from a non-polar to a polar solvent increased the solvent interaction and both the excitation and emission spectra were shifted to longer wavelength and intensity decreased through taking advantage of twisted intramolecular charge transfer (TICT). Density functional theory (DFT) calculations and spectral analyses revealed that such fluorophores were capable of sensing protons by intramolecular charge transfer (ICT). Empirical and quantum mechanical calculations showed that the electron donating effect of the dimethylamino group decreased the change in dipole moment on excitation which resulted in a fluoresc...

Longitudinal and transverse electromagnetic form factors of the 7Li ground-state doublet (the J?=3/2- ground state and the Ex=478 keV, J?=1/2- first excited state) were measured by electron scattering up to momentum transfers of 4.2 and 4.5 fm-1, respectively. The transverse form factors show no diffraction structures in the high momentum transfer region which could be unambiguously identified as signatures of meson-exchange currents; however, they lie above existing calculations which do not include such contributions. The longitudinal elastic form factor has a second maximum as does the C2 form factor of the first excited state.

Fluorescence-on sensors typically rely on disrupting photoinduced electron transfer quenching of the excited state through binding the electron donor. To provide a more general fluorescence-on signaling unit, a quencher-fluorophore dyad has been developed in which quenching by electron transfer to a tethered viologen acceptor can be disrupted through complexation of the viologen by cucurbit[7]uril (CB7). Dyads of benzyl viologen-rhodamine B or a BODIPY fluorophore gave upon CB7 complexation 14- and 30-fold fluorescence enhancement, respectively. PMID:22860771

Two-dimensional dipole-dipole energy transfer (Foerster type) has been studied with Langmuir-Blodgett (LB) monolayer films containing chromophores of carbazole (donor) and anthracene (acceptor). Fluorescence decay curves of donor (1.5 mol %) in the presence of acceptors (1.5-8.5 mol %) were measured with a picosecond time-correlated, single-photon-counting apparatus. The decay curves were analyzed in terms of a time-dependent equation of excited-state survival in the two-dimensional energy-transfer system. At substantially higher concentrations of acceptor (>5 mol %), the fluorescence decay follows the equation of two-dimensional energy-transfer kinetics plus an exponential decay term. At lower concentrations, the energy transfer competes with the fluorescence quenching due to the electron transfer from a photoexcited carbazole chromophore to carboxyl groups of stearic acid of the LB film. A nonexponential decay curve of carbazole chromophore without acceptor, which is independent of the concentration, is found to be consistent with a theoretical decay function of the electron-transfer quenching.

Reaction centers of bacterial photosynthesis are ideal systems to study photosynthetic energy conversion. Femtosecond spectroscopy has delivered extensive information on the molecular mechanisms of the primary electron transfer. The data show, that primary electron transfer is an ultrafast stepwise reaction, where the electron is transferred via closely spaced pigments with reaction times as fast as 0.9 ps and 3.5 ps. Experiments on mutated and modified reaction centers allow to determine the energetics of the various intermediates in the reaction center. Recently, femtosecond experiments with light pulses in the mid infrared have shown, that an additional fast process occurs on the 200 fs timescale in the initially excited special pair. Only afterwards the well established electron transfer reactions take place. This fast process may be of importance for the optimization of the primary reaction. {copyright} {ital 1996 American Institute of Physics.}

In order to ensure efficient utilization of the solar spectrum, photosynthetic organisms use a variety of antenna chromophores to absorb light and transfer excitation to a reaction center, where photoinduced charge separation occurs. Reported here is a synthetic molecular heptad that features two bis(phenylethynyl)anthracene and two borondipyrromethene antennas linked to a hexaphenylbenzene core that also bears two zinc porphyrins. A fullerene electron acceptor self-assembles to both porhyrins via dative bonds. Excitation energy is transferred very efficiently from all four antennas to the porphyrins. Singlet-singlet energy transfer occurs both directly and by a stepwise funnel-like pathway wherein excitation moves down a thermodynamic gradient. The porphyrin excited states donate an electron to the fullerene with a time constant of 3 ps to generate a charge-separated state with a lifetime of 230 ps. The overall quantum yield is close to unity. In the absence of the fullerene, the porphyrin excited singlet state donates an electron to a borondipyrromethene on a slower time scale. This molecule demonstrates that by incorporating antennas, it is possible for a molecular system to harvest efficiently light throughout the visible from ultraviolet wavelengths out to approximately 650 nm. PMID:19438278

We explore coherence dynamics in light-harvesting complexes and their interactions with other electronic states and vibrational modes. This is achieved by utilizing a two-colour four-wave mixing spectroscopy to excite and analyse a specific coherence pathway in the phycocyanin-645 (PC645) light-harvesting complex. We observe the dephasing rate increase as a function of temperature and oscillations in the signal intensity as a function of waiting time which reveals coherent excitation of pathways not directly resonant with the laser pulses. This coherent excitation of non-resonant electronic states implies strong coupling to phonon modes, which is necessary if coherent energy transfer between non-resonant states is to play any role in photosynthetic energy transfer.

Polymers with low optical gaps are of importance to the organic photovoltaics community due to their potential for harnessing a large portion of the solar energy spectrum. The combination along their backbones of electron-rich and electron-deficient fragments contributes to the presence of low-lying excited states that are expected to display significant charge-transfer character. While conventional hybrid functionals are known to provide unsatisfactory results for charge-transfer excitations at the time-dependent DFT level, long-range corrected (LRC) functionals have been reported to give improved descriptions in a number of systems. Here, we use such LRC functionals, considering both tuned and default range-separation parameters, to characterize the absorption spectra of low-optical-gap systems of interest. Our results indicate that tuned LRC functionals lead to simulated optical-absorption properties in good agreement with experimental data. Importantly, the lowest-lying excited states (excitons) are shown to present a much more localized nature than initially anticipated. PMID:22914764

Excited state dynamics of the highly stable 1:1 and 2:1 charge-transfer (CT) complexes assembled via host-guest interactions between a biscrown stilbene and a viologen vinylog was studied using transient pump-supercontinuum probe spectroscopy. In acetonitrile, both complexes showed ultrafast two-component transient absorption dynamics after excitation in the CT absorption band by a 616 nm, 70 fs laser pulse. The faster component ({tau}<200 fs) is assigned to relaxation processes in the lowest CT excited state. The second component is due to the back electron transfer (ET) reaction leading to the ground state. The measured ET time constants for the 1:1 and 2:1 CT complexes are about 540 fs and 1.08 ps, respectively. Excitation of the bimolecular complex by a 308 nm laser pulse gave rise to three-component transient absorption dynamics. The fastest transient ({tau}{approx}150 fs) is assigned to relaxation processes in the high-lying excited states of the complex. The high-amplitude rise component with a time constant of about 300 fs is due to the internal conversion from the high-lying excited states to the lowest CT excited state. The latter decayed to the ground state via the back ET with a time constant very close to that measured when the complex was excited in the CT absorption band.

A "frozen" electron donor-acceptor array that bears porphyrin and fullerene units covalently linked through the ortho position of a phenyl ring and the nitrogen of a pyrrolidine ring, respectively, is reported. Electrochemical and photophysical features suggest that the chosen linkage supports both through-space and through-bond interactions. In particular, it has been found that the porphyrin singlet excited state decays within a few picoseconds by means of a photoinduced electron transfer to give the rapid formation of a long-lived charge-separated state. Density functional theory (DFT) calculations show HOMO and LUMO to be localized on the electron-donating porphyrin and the electron-accepting fullerene moiety, respectively, at this level of theory. More specifically, semiempirical molecular orbital (MO) configuration interaction (CI) and unrestricted natural orbital (UNO)-CI methods shed light on the nature of the charge-transfer states and emphasize the importance of the close proximity of donor and acceptor for effective electron transfer. PMID:23018982

We propose a simple model to describe the dynamics of STM-induced desorption of a single CO molecule from Cu(111). In the model, a single electron initially occupies the CO 2?*-orbital. An excitation of the CO-Cu stretching vibration then occurs when this electron subsequently transfers/tunnels from the CO 2?*-orbital to the metal substrate. When the excitation of the CO-Cu stretching vibration exceeds some predesorption level in a truncated harmonic oscillator well, CO is desorbed from Cu(111). The results of our model calculation for the desorption probability agree quite well with recent experimental results.   

A mixed quantum-classical methodology is utilized to compute transient optical spectra reflecting excitation energy transfer in large pheophorbide-a complexes dissolved in ethanol. Room-temperature molecular dynamics simulations are used to describe the nuclear dynamics of the whole solvent-solute complex. The electronic excitation energy dynamics is accounted for by solving the time-dependent electronic Schrodinger equation. All computations are carried out in the framework of the ground-state classical path approximation and in the presence of optical excitations to determine the nonlinear response. A specification to a pump-probe scheme allows to determine spectra of transient anisotropy which offer signatures of a 10ps excitation energy redistribution already found in earlier studies (...

To realize the chemistry of a multicage organic molecule with excess electron, as a model, by confining an excess electron inside a double-cage single molecule, the structures of e?@C??F??(NH)?C??F?? (e?@AB) and e?@C??F??(NH)?C??F?? (e?@BB') are obtained at the B3LYP/6-31G(d) + 4s4p theory level. It is confirmed that the excess electron is mainly confined inside one cage with larger interior electronic attractive potential (A for e?@AB and B for e?@BB') in the ground state, while the electron is localized in the other one in the first excited state. Owing to such excess electron localizations, an interesting intercage excess electron transfer transition takes places. This intercage excess electron transfer transition exhibits five characteristics: (1) the excess electron transfer from one cage to another (A ? B for e?@AB and B ? B' for e?@BB?'); (2) the transition is between the ground and first excited state; (3) the wavelength and strength are the largest; (4) the transition accompanies a significant charge transfer (?q > 0.8) and molecular dipole moment change (?? > 20 D); (5) the transition corresponds to SOMO ? LUMO. For the transition, the oscillator strength is larger and the wavelength is shorter for the asymmetric structure (e?@AB) than for the symmetric one (e?@BB'), which indicates that the intercage excess electron transfer transition may be regulated by changing the size of cage. This work is useful for the designs of organic electronic sponges (porous organic electrides), organic conductor with excess electrons, and photoelectric and nanoelectronic devices. PMID:20882986

Dynamics of excited 5f electron states of the transuranium ions Cm{sup 4+} and Bk{sup 4+} in CeF{sub 4} are compared. Based on time- and wavelength-resolved laser-induced fluorescence, excitation energy transfer processes have been probed. Depending on concentration and electronic energy level structure of the studied 4+ transuranium ion, the dominant energy transfer mechanisms were identified as cross relaxation, exciton-exciton annihilation, and trapping. Energy transfer rates derived from the fitting of the observed fluorescence decays to theoretical models, based on electric multipolar ion-ion interactions, are contrasted with prior studies of 4f states of 3+ lanthanide and 3d states of transition metal ions. 16 refs., 1 tab.

The backscattering of energetic hydrogen particles, 1-1000 eV, from alkali metal surfaces provides for a relatively large yield of negative hydrogen ions. These yields are enhanced by particle reflection from surfaces consisting of partial alkali coatings over high-Z transition-metal substrates. The theoretical data supporting these observations are reviewed. The parameters leading to optimum reflection yields are summarized. In the volume of a hydrogen discharge with electron temperatures of about one electron volt, negative ions are formed by dissociative attachment to vibrationally excited molecules. The vibrational distribution is determined by e-V collisions between low energy electrons and vibrationally excited molecules, E-V singlet electron excitation processes caused by high energy (100 eV) electrons colliding with ground state molecules exciting to electronic states followed by radiative decay to higher vibrational levels, and V-T collisions between molecules resulting in transfer of vibrational excitation to translational energy. The role of these different processes as they bear on the vibrational distribution is discussed. The possibility of a volume-surface interaction leading to a high volume density of negative ions is considered.

9-Methylanthracene (9-MeAn) is found to function as an efficient, sequential energy and electron transfer shuttle between chromophore and redox sites bound to separate strands of chemically modified polystyrene. Sensitizied formation of the triplet excited state, /sup 3/(9-MeAn)*, occurs by energy transfer following visible irradiation of a polymer-bound polypyridyl complex of Ru(II), PS-(Ru/sup II/). In the presence of separate polymers containing reductive (phenothiazene, PS-PTZ) or oxidative sites (paraquat, PS-PQ/sup 2 +/), a series of subsequent electron transfer steps results in the transient generation of PS-PTZ/sup +/ and PS-PQ/sup +/ and the net photoinduced production and separation of oxidative reductive equivalents on separated polymers. The rate of recombination between PS-PQ/sup +/ and PS-PTZ/sup +/ by back electron transfer is reduced by a factor of 27 relative to an analogous system based on PTZ and PQ/sup 2 +/ monomers.

A numerical modeling of H2-F-HN3 and H2-F-NF2 flames was done in order to investigate the use of these flames as a pumping source for a N2(A)-IF electronic energy transfer (EET) laser. Equations for concentrations of different chemical species, populations of the ground and excited states of molecules, and energy-exchange processes were included in the model. The main processes determining the kinetics of electronically excited particles in H2-F-NF2 and H2-F-NH3 flames leading to the formation of N2(A) are listed. Results indicate the possibility of high concentrations of NA2(A) generation in these media. Electronically excited NA2(A) may be used as an energy donor to pump acceptors radiating in the visible region and the media considered appear to be promising sources for optical pumping.

The author gathers in this document several papers he has already published in order to shed light on different aspects concerning ion-crystal interactions. This document is divided into 3 chapters. In the first chapter the author presents results obtained from experiments dedicated to charge exchanges and energy released by heavy ions in channeling conditions. Different processes involved in ion-electron interactions are considered: The tri-electronic recombination, the electron capture through nuclear excitation (NEEC), resonant transfer and excitation (RTE), resonant transfer and double excitation (RTDE) and electron impact ionization (EII). The second chapter deals with the measurement of nuclear fission times through crystal blocking experiments. The crystal blocking technique allows the measurement in a model-independent way of the recoil distance covered by the excited nucleus during the whole fission process (starting from the initial collision and ending at the scission point). The last chapter is dedicated to the photon impact ionization through the conversion of a high-energy photon into an electron-positron pair.

We discuss the production of ultracold molecules in their electronic ground state by photoassociation employing electronically excited states with ion-pair character and strong spin-orbit interaction. A short photoassociation laser pulse drives a nonresonant three-photon transition for alkali-metal atoms colliding in their lowest triplet state. The excited-state wave packet is transferred to the ground electronic state by a second laser pulse, driving a resonant two-photon transition. After analyzing the transition matrix elements governing the stabilization step, we discuss the efficiency of population transfer using transform-limited and linearly chirped laser pulses. Finally, we employ optimal control theory to determine the most efficient stabilization pathways. We find that the stabilization efficiency can be increased by one and two orders of magnitude when using linearly chirped and optimally shaped laser pulses, respectively.

We report the results of variational calculations on low energy {ital e}{sup -}+HBr collisions using the complex Kohn method. We compare the results of all-electron numerical calculations with those in which effective core potentials are used. We present total, differential, and momentum transfer cross sections for electronically elastic scattering, as well as dissociative excitation cross sections for the low-lying electronic states that dissociate to ground-state neutral atoms. We find excellent agreement between the all-electron and core-potential results for all processes considered. {copyright} {ital 1996 American Institute of Physics.}

A discrete charge transfer in a small tunnel junction where Coulomb interactions are important can excite electron-hole pairs near the Fermi level. We use a simple model to study the associated nonequilibrium properties and found two novel effects: (i) for junctions with electrodes of the same electronic properties, a leakage current exists within the Coulomb gap even when the environmental impedance is infinite; (ii) for junctions with electrodes of different electronic properties, the differential conductance diverges when a net interaction between conduction electrons is attractive, and it is strongly suppressed for a net repulsive interaction.

We study theoretically the electronic and nuclear dynamics in NaI. After a femtosecond pulse has prepared a wave packet in the first excited state, we consider the adiabatic and the nonadiabatic electronic dynamics and demonstrate explicitly that a nonstationary electron is created in NaI corresponding to electron transfer between Na and I. The electronic motion is introduced via nuclear motion, more specifically, through nonadiabatic curve crossing and the electronic motion is here on the same time scale as the nuclear motion. We show that the branching ratio between the channels Na + I and Na+ + I- depends on the electron distribution (i.e., where the electron "sits") prior to the time where the bond is broken by a subpicosecond half-cycle unipolar electromagnetic pulse. Thus we control, in real time, which nucleus one of the valence electrons will follow after the bond is broken. (C) 1998 American Institute of Physics.

Optical excitation of the sequential supermolecule H_2P-ZnP-Q induces an electron transfer from the free-base porphyrin (H_2P) to the quinone (Q) via the zinc porphyrin (ZnP). This process is modeled by equations of motion for the reduced density matrix which are solved numerically and approximately analytically. These two solutions agree very well in a great region of parameter space. It is shown that for the majority of solvents the electron transfer occurs with the superexchange mechanism.

Photosynthetic electron transfer is arguably the most important series of chemical transformations for life on this planet. In recent years the structure of the reaction centers (RC) from the photosynthetic bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides have been presented. On the basis of these structures, several mechanisms have been proposed to explain the primary electron-transfer event with as yet no consensus. The authors report here INDO/S calculations of the excited states of a model of the RC of Rps. viridis in both the absence and presence of a polarizable medium.

A series of pseudo-tetrahedral copper(I) complexes carrying bis(imino)acenaphthene (BIAN) ligands as acceptor subunits and various phosphane derivatives was prepared and characterized by elemental analysis, X-ray crystallography and spectroscopic techniques. The electronic spectra of the compounds are dominated by low-lying metal-to-ligand charge transfer (MLCT) transitions which could be systematically modified by different substituent patterns at the diimine acceptor subunit and by variations of the electron donating properties and bite angles of the phosphane moiety. A qualitative model based on frontier-orbital overlap arguments is introduced to describe the observed variations in optical spectra, excited state energies, solvatochromic behaviour, charge transfer character, and extent o...

The time-development of photoexcitations in molecular aggregates exhibits specific dynamics of electronic states and vibrational wavefunction. We discuss the dynamical formation of entanglement between electronic and vibrational degrees of freedom in molecular aggregates with theory of electronic energy transfer and the method of vibronic 2D wavepackets [Cina, Kilin, Humble, J. Chem. Phys. 118, 46 (2003)]. The vibronic dynamics is also described by applying Jaynes-Cummings model to the electronic energy transfer [Kilin, Pereverzev, Prezhdo, J. Chem. Phys. 120, 11209 (2004);math-ph/0403023]. Following the ultrafast excitation of donor[chem-ph/9411004] the population of acceptor rises by small portions per each vibrational period, oscillates force and back between donor and acceptor with later damping and partial revivals of this oscillation. The transfer rate gets larger as donor wavepacket approaches the acceptor equilibrium configuration, which is possible at specific energy differences of donor and acceptor...

The SAC-CI method has been applied to the theoretical spectroscopy of the inner-shell electronic processes and the photochemistry of the organic light-emitting diodes (OLED) and biological chemosensors. Wide varieties of the core-electronic processes such as core-electron ionizations, shake-up satellites, vibrational excitations, valence-Rydberg coupling, and its thermal effect have been investigated by the SAC-CI calculations. The method has also been applied to the electronic spectra and the excited-state dynamics of the polymer materials of OLED such as poly para-phenylene vinylene and fluorene-thiophene. The photochemistry of the biological chemosensor has been elucidated in particular for the photo-induced electron transfer mechanism of the acridine-type fluorescent probe.

The development of X-ray and electron diffraction methods with ultrahigh time resolution has made it possible to map directly, at the atomic level, structural changes in solids induced by laser excitation. This has resulted in unprecedented insights into the lattice dynamics of solids undergoing phase transitions. In aluminium, for example, femtosecond optical excitation hardly affects the potential energy surface of the lattice; instead, melting of the material is governed by the transfer of thermal energy between the excited electrons and the initially cold lattice. In semiconductors, in contrast, exciting approximately 10 per cent of the valence electrons results in non-thermal lattice collapse owing to the antibonding character of the conduction band. These different material responses raise the intriguing question of how Peierls-distorted systems such as bismuth will respond to strong excitations. The evolution of the atomic configuration of bismuth upon excitation of its A(1g) lattice mode, which involves damped oscillations of atoms along the direction of the Peierls distortion of the crystal, has been probed, but the actual melting of the material has not yet been investigated. Here we present a femtosecond electron diffraction study of the structural changes in crystalline bismuth as it undergoes laser-induced melting. We find that the dynamics of the phase transition depend strongly on the excitation intensity, with melting occurring within 190 fs (that is, within half a period of the unperturbed A(1g) lattice mode) at the highest excitation. We attribute the surprising speed of the melting process to laser-induced changes in the potential energy surface of the lattice, which result in strong acceleration of the atoms along the longitudinal direction of the lattice and efficient coupling of this motion to an unstable transverse vibrational mode. That is, the atomic motions in crystalline bismuth can be electronically accelerated so that the solid-to-liquid phase transition occurs on a sub-vibrational timescale. PMID:19262668

Photochemical and photophysical properties of the triplet excited states of dendritic multiporphyrins arrays (nPZn, n = 1, 3, and 7) have been investigated by measuring the nanosecond transient absorption spectra in the visible and near-IR regions with changing the generation number. Intermolecular triplet–triplet annihilation rates decrease with the dendrimer generation, which was interpreted on a proposed kinetic model assuming that the excited triplet energy almost localizes in one PZn unit in nPZn. In the presence of C60, intermolecular electron-transfer takes place via the excited triplet states of nPZn (3nPZn*), yielding the cation radical of nPZn (nPZn•+) and the anion radical of C60 (C60•?) in PhCN. Deceleration of the electron-transfer rate-constants from 1PZn to 3PZn and the acceleration from 3PZn to 7PZn were observed, in which the latter tendency was interpreted by considering an increase in effective encounter radius for 37PZn*. The observed small change of the rate constants for back electron transfer between the oppositely charged species with the dendrimer generation was also reasonably interpreted by taking a smaller effective radius due to electrostatic attraction into consideration. Dendrimer generation effect was also observed for the intermolecular hole-transfer process.   

We have investigated the energy transfer dynamics in a supramolecular linear polymer chain comprising oligofluorene (OF) energy donor units linked by quadruple hydrogen-bonding groups, and oligophenylene (OPV) chain ends that act as energy acceptors. Using femtosecond spectroscopy, we followed the dynamics of energy transfer from the main chain of OF units to the OPV chain ends and simulated these data taking a Monte Carlo approach that included different extents of electronic wave function delocalization for the energy donor and acceptor. Best correlations between experimental and theoretical results were obtained for the assumption of electronic coupling occurring between a localized donor dipole moment and a delocalized acceptor moment. These findings emphasize that geometric relaxation following initial excitation of the donor needs to be taken into account, as it leads to a localization of the donor's excited state wave function prior to energy transfer. In addition, our simulations show that the energy transfer from the main chain to the ends is dominated by an interplay between slow and spatially limited exciton migration along the OF segments comprising the main chain and the comparatively faster hetero-transfer to the end-cap acceptors from directly adjoining OF segments. These results clearly support the description of host-guest energy transfer in linear polymer chains as a two-step mechanism with exciton diffusion in the host being a prerequisite to energy transfer to the guest. PMID:22753826

This paper contains viewgraphs on the SuperHILAC. The topics of these viewgraphs are: light charged particle emission as a probe of heavy-ion reactions; correlated charged-changing interactions and x-ray emission in ion-atom collisions; progress report on Sassy II and new nuclear chemistry experiments at the SuperHILAC; precision x-ray spectroscopy of heavy ions; 180/sup 0/-correlated equal energy photons from 5.9 MeV/N U + Th collisions; research statement of excited states of monatomic and molecular systems; search for entrance-channel effects in the production of superdeformed nuclei; present and future research with OASIS; relaxation mechanisms in damped heavy-ion reactions; excitation energy division and nucleon transfer; test of QED and relativistic effects for strongly-bound electrons; heavy-ion Coulomb excitation and transfer reactions as probes of nuclear structure; and preliminary design of the Dilepton spectrometer.

Resonant soft X-ray emission spectroscopy has been demonstrated to possess interesting abilities for studies of electronic structure in various systems, such as symmetry probing, alignment and polarization dependence, sensitivity to channel interference, etc. In the present abstract the authors focus on the feasibility of resonant soft X-ray emission to probe low energy excitations by means of resonant electronic X-ray Raman scattering. Resonant X-ray emission can be regarded as an inelastic scattering process where a system in the ground state is transferred to a low excited state via a virtual core excitation. The energy closeness to a core excitation of the exciting radiation enhances the (generally) low probability for inelastic scattering at these wavelengths. Therefore soft X-ray emission spectroscopy (in resonant electronic Raman mode) can be used to study low energy d-d excitations in transition metal systems. The involvement of the intermediate core state allows one to use the selection rules of X-ray emission, and the appearance of the elastically scattered line in the spectra provides the reference to the ground state.

Carbon nanotubes (CNTs) coated with nanocrystals (NCs) or quantum dots (QDs) form a new class of hybrid nanomaterials that could potentially display both the unique properties of NCs and those of CNTs. The photo-induced charge transfer within the hybrid nanostructure may either increase or decrease the CNT electrical conductivity. The use of a chemical vapor deposition method realizes direct assembly of CdSe NCs onto the external surfaces of single-walled CNTs without applying any organic linkers. The NC loading on the nanotubes can be tuned simply through deposition duration. Upon visible light excitation, photo-generated electrons are injected into CNTs from the excited states of CdSe NCs, which is monitored by the measurement of the photocurrent and field effect transistor (FET) characteristics. The direction of the electron transfer could be controlled with the NC coverage on CNTs and the gate voltage.Carbon nanotubes (CNTs) coated with nanocrystals (NCs) or quantum dots (QDs) form a new class of hybrid nanomaterials that could potentially display both the unique properties of NCs and those of CNTs. The photo-induced charge transfer within the hybrid nanostructure may either increase or decrease the CNT electrical conductivity. The use of a chemical vapor deposition method realizes direct assembly of CdSe NCs onto the external surfaces of single-walled CNTs without applying any organic linkers. The NC loading on the nanotubes can be tuned simply through deposition duration. Upon visible light excitation, photo-generated electrons are injected into CNTs from the excited states of CdSe NCs, which is monitored by the measurement of the photocurrent and field effect transistor (FET) characteristics. The direction of the electron transfer could be controlled with the NC coverage on CNTs and the gate voltage. Electronic supplementary information (ESI) available. See DOI: 10.1039/c2nr11577h

The electronic structures and relative stabilities of homopolar biradicals (BR) and CT BR with significant one-electron transfer (ET) BR characters were investigated by ab initio MO calculations. The previously presented inter- and intra-molecular CT models were extended in order to elucidate possible mechanisms for decomposition reactions of dioxetane, dioxetanone, and related species. The computational results indicate that endothermic O–O cleavages, followed by charge-transfers, are operative for the chemiluminescence reactions of these peroxides with several anionic species, in contradiction to the chemically initiated electron-exchange luminescence (CIEEL) mechanism, where complete one-electron transfer (ET) is required for the formation of excited carbonyl fragments. The ionization potentials of monoanions of phenol, indole and luciferins were calculated semiempirically in order to estimate the CT excitation energies from these species to the O–O antibonding orbital. The CT excitation energies are used to distinguish between the CT induced luminescence (CTIL) mechanism and the CIEEL mechanism for chemiluminescence reactions. Orbital-interaction models are also presented to explain of the so-called odd/even selection rule for the efficiency of chemiluminescence reactions. The implications of these results are discussed in relation to recent experimental results, together with biological chemiluminescence reactions of luciferins and related species.   

We propose a first-principles method for evaluations of the time-dependent electron distribution function of excited electrons in the conduction band of semiconductors. The method takes into account the excitations of electrons by an external source and the relaxation to the bottom of the conduction band via electron-phonon coupling. The methods permit calculations of the non-equilibrium electron distribution function, the quasi-stationary distribution function with a steady-in-time source of light, the time of setting of the quasi-stationary distribution and the time of energy loss via relaxation to the bottom of the conduction band. The actual calculations have been performed for titanium dioxide in the anatase structure and zinc oxide in the wurtzite structure. We find that the quasi-stationary electron distribution function has a peak near the bottom of the conduction band and a tail whose maximum energy rises linearly with increasing energy of excitation. The calculations demonstrate that the relaxation of excited electrons and the setting of the quasi-stationary distribution occur within a time of no more than 500 fs for ZnO and 100 fs for anatase. We also discuss the applicability of the effective phonon model to energy-independent electron-phonon transition probability. We find that the model only reproduces the trends in the change of the characteristic times whereas the precision of such calculations is not high. The rate of energy transfer to phonons at the quasi-stationary electron distribution also have been evaluated and the effect of this transfer on the photocatalysis has been discussed. We found that for ZnO this rate is about five times less than in anatase.

The electronic structure and some important intra-molecular charge transfer parameters were investigated at CAS-SCF, MRCI, CAS+S and multi-reference localization levels of theory for purely organic mixed-valence molecules. In particular, a spiro cation has been taken as a model system. The potential energy surfaces of the ground and the lower three excited electronic states have been computed within a two-state model, at CAS-SCF using TZP basis for the spiro cation, and an adiabatic double-well potential has been obtained for the ground electronic state. Our analysis of the geometry through the reaction coordinate indicate that the spiro cation is a valence trapped bistable system. The effect of non-dynamical correlation, using a localized orbital approach, was found to be crucial for a quantitative description of the electronic structure and some important electron transfer parameters of these organic mixed-valence systems.

The ground and excited state geometries, the excitation and emission energies for a series of fluorescein derivatives in aqueous solutions have been investigated using time-dependent density functional theory (TD-DFT) with B3LYP and a long-range corrected CAM-B3LYP functional. The RI-CC2 method was employed to confirm the CAM-B3LYP results. As far as we know, the excited state geometries for a series of fluorescein derivatives were optimized for the first time, and the conformational changes upon photoexcitation were discussed. Importantly, the previous proposed photo induced electron transfer (PeT) mechanism for dictating the fluorescence quantum yield (?(fl)) of fluorescein derivatives was not fully supported by our calculations. Internal conversion may still be the most likely mechanism for dictating the ?(fl) of fluorescein derivatives, which indicates a need for further experimental and theoretical studies on the excited state dynamics of fluorescein derivatives. PMID:23042032

We use molecules to couple light into and out of microscale plasmonic waveguides. Energy transfer, mediated by surface plasmons, from donor molecules to acceptor molecules over ten micrometer distances is demonstrated. Also surface plasmon coupled emission from the donor molecules is observed at similar distances away from the excitation spot. The lithographic fabrication method we use for positioning the dye molecules allows scaling to nanometer dimensions. The use of molecules as couplers between far-field and near-field light offers the advantages that no special excitation geometry is needed, any light source can be used to excite plasmons and the excitation can be localized below the diffraction limit. Moreover, the use of molecules has the potential for integration with molecular electronics and for the use of molecular self-assembly in fabrication. Our results constitute a proof-of-principle demonstration of a plasmonic waveguide where signal in- and outcoupling is done by molecules.

We concentrate on several relatively new aspects of the study of fast electron scattering by atoms and atom-like objects, namely endohedral atoms and fullerenes. We show that the corresponding cross sections, being expressed via so-called Generalized Oscillator Strengths (GOS), give information on the electronic structure of the target and on the role of electron correlations in it. We consider what sort of information became available when analyzing the dependence of GOS upon their multipolarity, transferred momentum and energy. We demonstrate the role of nondipole corrections in the small-angle fast-electron inelastic scattering. There dipole contribution dominates while non-dipole corrections can be considerably and controllably enhanced as compared to the case of low and medium energy photoionization. We show also that analyses of GOS for discrete level excitations permit to clarify their multipolarity. The results of calculations of Compton excitation and ionization cross-sections for noble gas atoms are...

In this work we present a systematic study of three representative iridium dyes, namely, Ir(ppy)3, FIrpic, and PQIr, which are commonly used as sensitizers in organic optoelectronic devices. We show that electronic correlations play a crucial role in determining the excited-state energies in these systems, due to localization of electrons on Ir d orbitals. Electronic localization is captured by employing hybrid functionals within time-dependent density-functional theory and with Hubbard-model corrections within the ?-SCF approach. The performance of both methods are studied comparatively and shown to be in good agreement with experiment. The Hubbard-corrected functionals provide further insight into the localization of electrons and on the charge-transfer character of excited-states. The gained insight allows us to comment on envisioned functionalization strategies to improve the performance of these systems. Complementary discussions on the ?-SCF method are also presented in order to fill some of the gaps in the literature.

Bacterial photosynthetic reaction centers (RC) contain eight chromophores forming a well-defined supramolecular structure within a protein framework. Theoretical studies suggest that the excited states of these chromophores are delocalized and contain important contributions from charge-transfer and resonance states. There is no clear-cut experimental evidence pertaining to the degree of localization of excited states. We have used ultrafast near and mid-infrared spectroscopic methods to investigate the character of some of the excited states. Exciting the 800 nm, absorption band, we followed the fate of the excitation energy using either the stimulated emission of the special pair at 920 nm or a transient absorption at 1.2 {mu}m. For a completely localized system, Forster theory-based calculations are expected to accurately predict the kinetics of energy transfer. It was found, however, that calculated rates arc much faster than measured rates. This corroborates a delocalized picture, with internal conversion rather than energy transfer between states. We have also measured the transient absorption spectrum of the RC in the infrared spectral region, detecting several new low-lying electronic states. Assignments for these states, and implications for the localization problem will be discussed.

The optical reflectivity spectrum and its temperature dependence were measured on a cleaved (001) surface of SmBaCo2O5.6. With an increase in temperature, the reflectivity below 1eV increases up to the metal-insulator transition temperature, TMI=360K, and saturates above TMI. The spectrum of optical conductivity ?(?) shows large variation in temperature which accompanies the transfer of spectral weight between a broad peak around 3eV, which can be assigned to the O2p–Co3d charge-transfer excitation, and the incoherent excitation of carriers below 1eV. The transferred spectral weight is as large as 0.17 per Co ion, which implies that the effective mass of an electron, m*, can be estimated to be 6m0. The optical gap of 0.2eV at 20K is consistent with the resistivity measurement.   

The photophysics of a red-emitting push-pull triarylamine compound (fvin) comprising a triphenylamino electron donor core and a dicyanovinylene electron acceptor group is investigated by steady-state absorption and emission and femtosecond time-resolved absorption spectroscopy in room-temperature n-hexane, toluene, ethanol, and acetonitrile solvents. Fvin is strongly fluorescent in apolar solvents upon excitation of the S0->S1 intramolecular charge transfer (ICT) transition, but hardly emissive in polar solvents. Time-resolved spectra reveal a strong dependence of the excited-state dynamics on both the solvent polarity and viscosity. Unlike n-hexane solutions where the fluorescent ICT excited state remains weakly solvated and keeps a structure close to that of the Franck-Condon level, sign...

Abstract A theoretical and experimental investigation of meta-aminobenzoic acid (MABA) in the gas phase is presented, with the goal of understanding counterintuitive observations on the solvatochromism of this -push-pull- molecule. The adiabatic excitation energies, transition moments, and excited-state structures are examined using the complete active space self-consistent field approach (CASSCF and CASPT2), which shows the first excited electronic state of MABA to be of greater charge transfer character than was found in the para-isomer (PABA). The rotationally resolved electronic spectrum of MABA reveals the existence of two rotamers, owing to asymmetry in the carboxylic acid functional group. Stark measurements in a molecular beam show the change in permanent dipole moment upon excitat...

A theoretical and experimental investigation of meta-aminobenzoic acid (MABA) in the gas phase is presented, with the goal of understanding counterintuitive observations on the solvatochromism of this "push-pull" molecule. The adiabatic excitation energies, transition moments, and excited-state structures are examined using the complete active space self-consistent field approach (CASSCF and CASPT2), which shows the first excited electronic state of MABA to be of greater charge transfer character than was found in the para-isomer (PABA). The rotationally resolved electronic spectrum of MABA reveals the existence of two rotamers, owing to asymmetry in the carboxylic acid functional group. Stark measurements in a molecular beam show the change in permanent dipole moment upon excitation to be ???3.5 D for both rotamers, more than three times larger than that found in PABA. The excited state measurements reported here, along with supporting data from theory, clearly demonstrate how the meta-directing effects of asymmetric substitution in aniline derivatives can drive charge transfer pathways in the isolated molecule. PMID:21557435

Oscillator strengths for C 1s, N 1s, and O 1s excitation spectra of aniline, nitrobenzene, and isomeric nitroanilines have been derived from inner-shell electron energy loss spectroscopy recorded under low momentum transfer conditions. Extended Huckel Molecular Orbital (EHMO) calculations carried out within the equivalent core analogy are used to aid spectral interpretation. Strong multielectron excitation features were not found, although these have been expected from emission studies. Spectral features of the nitroanilines have been found to be strongly dependent on the substitution pattern (i.e. whether ortho, meta, or para). 48 refs., 11 figs.

In an effort to facilitate water photosplitting at surfaces, we identify quantum well states of magic gold clusters supported on ultrathin MgO/Ag(001) as the key to favor sunlight absorption and photocatalytic reactions. Based on density functional theory (DFT) and time-dependent DFT calculations, the adsorption geometry, electronic structures, and excited state properties of supported metal nanoparticles can be precisely controlled. By decreasing the thickness of MgO film, charge transfer to supported gold clusters, and therefore the occupation and energy spacings of quantum well states, can be gradually tuned, leading to redshifted and enhanced plasmonic excitations and optimized energy levels for water splitting.

In high-resolution resonant inelastic x-ray scattering at the Ti L edge of the charge-density-wave system 1T-TiSe(2), we observe sharp low energy loss peaks from electron-hole pair excitations developing at low temperature. These excitations are strongly dispersing as a function of the transferred momentum of light. We show that the unoccupied bands close to the Fermi level can effectively be probed in this broadband material. Furthermore, we extract the order parameter of the charge-density-wave phase from temperature-dependent measurements. PMID:23006106

The two-magnon (2M) excitation at 3000 cm-1 in Sr_14Cu_24O_41 two-leg ladder is studied by Raman scattering. A slight anisotropy of the superexchange coupling J_{\\perp}/J_{||} \\approx 0.8 with J_{||} = 110 \\pm 20 meV is proposed from the analysis of the magnetic scattering. The resonant coupling across the charge transfer gap increases the 2M intensity by orders of magnitude. The anisotropy of Raman scattering is dependent upon the excitation energy. The 2M relaxation is found to be correlated with the temperature dependent electronic Raman continuum at low frequencies.

We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected using the time-resolved electron paramagnetic resonance (TREPR) method in the photosystem II center of the plants. We show, using the stochastic Liouville equation, that the ESP pattern created in the accessory chlorophyll (ChlaccD1) which reside near the PD1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model, initiating from the singlet?triplet conversion of the light-induced charge-separated state composed of PD1 radical cation and pheophytin radical anion. We also considered the sequential ESP transfer model via the triplet charge-recombination (CR) and the triplet?triplet energy transfer processes. It ...

Interfacial electron transfer reaction mechanism has been probed using l = 254 nm excited TiO2 nanoparticles and cis-[CoIII(en)2(RNH2)Cl]Cl2 adsorbates (where RNH2 = MeNH2, EtNH2, PrnNH2, BunNH2, BuiNH2, PennNH2, HexnNH2, BznNH2) in aqueous 2-propanol. These tailor made complexes differing in coordination environment due to RNH2 adhere onto TiO2 surface producing compact nano-TiO2//cobalt(III)-(RNH2) surface compound. The surface of the anatase under UV irradiation is uniquely powerful as adsorbent due to inherent hydrophobic/hydrophilic properties. Therefore, the compact structure facilitates an efficient electron transfer to the Co(III) center resulting a high photoefficiency of formation of Co(II). A model for the electron transfer is arrived by considering: (i) the overlap of conductio...

Polarized high resolution absorption spectra at 4.2 K are reported for single crystals of Cs/sub 2/U(Np)O/sub 2/Cl/sub 4/ and CsU(Np)O/sub 2/(NO/sub 3/)/sub 3/. Five transitions of the charge-transfer type are observed in both complexes. Zeeman effect measurements give values of g(parallel) in every case. MCD measurements in the nitrate complex permit the unambiguous assignment of a number of excited state symmetries. It is shown by evaluation of the electron-electron repulsion integrals that the excited states are best described as originating from sigmasub(u)..gamma gamma..', as opposed to ..pi..sub(u)/sup 3/..gamma gamma..', configurations. These configurations are analogous to those describing the charge-transfer states of the uranyl(VI) ion and in some cases the parentage of the neptunyl transitions can be recognized from the polarization of their vibronic structure.

Ab initio calculations (coupled cluster with single and double excitations; CCSD) have been used to investigate the model redox systems ethylene:M(0) (M = Li, Na, K, Rb, Cs) and ethylene:M(I) (M = Be, Mg, Ca, Sr, Ba, Zn, Cd, Hg). Within C2v symmetry, the ground (2A1) states correspond to the charge distribution given in the title. The lowest (2B2) excited states correspond, somewhat counter intuitively, to the ethylene*-/=M(II)ion pair. These trends can be rationalized on the basis of simple electrostatic and configuration-mixing arguments that lead to two simple equations for predicting the electron-transfer energies for oxidation or reduction of the ethylene. The electron-transfer energies to the 2B2 ion pairs are dominated by the electrostatic ion-pairing energies. PMID:16341719

In the photoinduced intramolecular electron transfer events of donor–bridge–acceptor molecular systems, the bridges play important roles. In the first part of this account, we review charge separation and charge recombination of porphyrin–bridge–fullerene systems, in which the porphyrin acts as a photosensitizing electron donor, whereas the ground state of the C60 moiety acts as an electron acceptor. In such systems, the charge separation usually takes place through the LUMO of the bridges via a super-exchange mechanism and/or a hopping mechanism, depending on their relative LUMO energy levels. On the other hand, the charge recombination of the radical ion pairs usually takes place through the HOMO of the bridges. In the second part, research of charge separation via the excited state of fullerene through the HOMO of the bridges is reviewed. So far, very small damping factors have been reported for ?-conjugated bridges such as oligophenyleneacetylenes, oligothiophenes, and oligothiophenevinylenes. On summarizing these results, it is revealed that switching from the super-exchange mechanism in short bridges to hopping mechanism in longer bridges is important to achieve long distance electron transfer through the bridges. Roles of molecular wires connecting electron donors such as porphyrin and electron acceptors such as fullerene in photosensitizing electron-transfer processes are summarized in this account. High electron-transferring abilities of the ?-conjugated molecular wires are revealed in connection with the MO energy levels and shape of the LUMO and HOMO of these molecular wires.   Fullsize Image

This thesis consists of two separate parts. The first part addresses the synthesis and study of conjugated polymers containing di- or triphenylamine. Two types of polymers: linear polymers and dendrimers, were synthesized. The polymers were characterized by NMR, IR, UV, GPC, TGA and DSC. Electronic and optical properties of the polymers were studied through the conductivity measurements and excitation- emission spectra. the second part of this thesis deals with a reaction of electron-rich acetylenes with TCNE. The discovery of the reaction from charge transfer complex studies and the investigation of this reaction on various electron-rich acetylenes are presented.

Transient absorption spectroscopy is used to study the excited-state dynamics of Co(3)(dpa)(4)(NCS)(2), where dpa is the ligand di(2-pyridyl)amido. The pi pi*, charge-transfer, and d-d transition states are excited upon irradiation at wavelengths of 330, 400 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution and the transient species show weak absorption. We thus propose that a low-lying metal-centered d-d state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal rapid conversion from the ligand-centered pi pi* and the charge-transfer states to this metal-centered d-d state within 100 fs. The excited molecule then crosses to a second d-d state within the ligand-field manifold, with a time coefficient of 0.6-1.4 ps. Because the ground-state bleaching band recovers with a time coefficient of 10-23 ps, we propose that an excited molecule crosses from the low-lying d-d state either directly within the same spin system or with spin crossing via the state (2)B to the ground state (2)A(2) (symmetry group C(4)). In this trimetal string complex, relaxation to the ground electronic surface after excitation is thus rapid. PMID:20049764

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 {nu} ([chromophores]/[micelle]), but progressively worse as {nu} 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 {nu} investigated and provides a determination of the second virial coefficient. 34 refs., 4 figs., 2 tabs.

Recent experimental data point to an asymmetric ground-state electronic distribution in the special pair (P) of purple bacterial reaction centers, which acts as the primary electron donor in photosynthesis. We have performed a density functional theory investigation on an extended model including the bacteriochlorophyll dimer and a few relevant surrounding residues to explore the origin of this asymmetry. We find strong evidence that the ground-state electron density in P is intrinsically asymmetric due to protein-induced distortions of the porphyrin rings, with excess electron charge on the P(M) bacteriochlorophyll cofactor. Moreover, the electron charge asymmetry is strongly modulated by the specific orientation of the C3(1) acetyl group, which is hydrogen bonded to His168. The electronic excitation has a significant charge transfer character inducing a displacement of electron charge from P(L) to P(M), in agreement with experimental data in the excited state. These results are relevant for the understanding of the unidirectional electron transfer path in photosynthesis. PMID:21512686

A homologous series of four molecules in which a phenol unit is linked covalently to a rhenium(I) tricarbonyl diimine photooxidant via a variable number of p-xylene spacers (n = 0-3) was synthesized and investigated. The species with a single p-xylene spacer was structurally characterized to get some benchmark distances. Photoexcitation of the metal complex in the shortest dyad (n = 0) triggers release of the phenolic proton to the acetonitrile/water solvent mixture; a H/D kinetic isotope effect (KIE) of 2.0 ± 0.4 is associated with this process. Thus, the shortest dyad basically acts like a photoacid. The next two longer dyads (n = 1, 2) exhibit intramolecular photoinduced phenol-to-rhenium electron transfer in the rate-determining excited-state deactivation step, and there is no significant KIE in this case. For the dyad with n = 1, transient absorption spectroscopy provided evidence for release of the phenolic proton to the solvent upon oxidation of the phenol by intramolecular photoinduced electron transfer. Subsequent thermal charge recombination is associated with a H/D KIE of 3.6 ± 0.4 and therefore is likely to involve proton motion in the rate-determining reaction step. Thus, some of the longer dyads (n = 1, 2) exhibit photoinduced proton-coupled electron transfer (PCET), albeit in a stepwise (electron transfer followed by proton transfer) rather than concerted manner. Our study demonstrates that electronically strongly coupled donor-acceptor systems may exhibit significantly different photoinduced PCET chemistry than electronically weakly coupled donor-bridge-acceptor molecules. PMID:22809316

The objective of this contract was the study of state-to-state, electronic energy transfer reactions relevant to the excited state chemistry observed in discharges. We studied deactivation reactions and excitation transfer in collisions of excited states of xenon and krypton atoms with Ar, Kr, Xe and chlorine. The reactant states were excited selectively in two-photon transitions using tunable u.v. and v.u.v. lasers. Excited states produced by the collision were observed by their fluorescence. Reaction rates were measured by observing the time dependent decay of signals from reactant and product channels. In addition we measured interaction potentials of the reactants by laser spectroscopy where the laser induced fluorescence or ionization is measured as a function of laser wavelength (excitation spectra) or by measuring fluorescence spectra at fixed laser frequencies with monochromators. The spectra were obtained in the form of either lineshapes or individual lines from rovibrational transitions of bound states. Our research then required several categories of experiments in order to fully understand a reaction process: 1. High resolution laser spectroscopy of bound molecules or lineshapes of colliding pairs is used to determine potential curves for reactants. 2. Direct measurements of state-to-state reaction rates were measured by studying the time dependent loss of excited reactants and the time dependent formation of products. 3. The energy selectivity of a laser can be used to excite reactants on an excited surface with controlled internuclear configurations. For free states of reactants (as exist in a gas cell) this has been termed laser assisted reactions, while for initially bound states (as chemically bound reactants or dimers formed in supersonic beams) the experiments have been termed photo-fragmentation spectroscopy.

Several new derivatives of 1,8-naphthalimide, which are connected with 1,3,4-oxadiazole via a covalent bond, have been synthesized. We have investigated the electroluminescent (EL) device fabricated with these new compounds. In these compounds 1,3,4-oxadiazole moiety facilitates the electron injection from the electrode and transfers its excited energy to 1,8-naphthalimide which acts as an emitting center. Greenish yellow EL peaked at 532 nm with a maximum luminous efficiency of 0.43 lm/W was observed.   

We study physical properties of photogenerated electron-lattice coupled states, polarons, in one-dimensional (1D) Peierls-Hubbard model with classical lattice distortion by means of the density matrix renormalization group method. The numerical results show novel midgap peaks in optical response spectra of polarons for large on-site Coulomb interaction U. These midgap peaks originate from charge-transfer excitations within the dimer in polarons.

Nanosecond laser flash photolysis technique was used to study photochemistry of Fe(III) complex with glioxalic acid. The primary photochemical process was found to be inner-sphere electron transfer in the excited complex leading to formation of the long-lived radical complex [FeII ? ?OOC-C(O)H]2+. A number of important spectral-kinetic parameters of this species were determined and mechanism of photolysis of Fe(III)-glioxalate complex was proposed. 1 The article was translated by the authors.

Tris(bipyridyl)ruthenium(II) ([Ru(bpy)3]2+: TB(II)) catalyzes oxidative coupling of enamines and aldehydes with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) under irradiation of visible light to afford ?-oxyaminated carbonyl compounds. The visible light irradiation is essential to generate the triplet excited state of ?TB(II) which acts as an oxidizing agent. This is a new procedure for radical coupling based on single electron transfer mediated by photoactivated TB.   

Momentum-resolved resonant inelastic x-ray scattering (RIXS) spectroscopy has been carried out successfully at the Fe K-edge for the first time. The RIXS spectra of a FeBO{sub 3} single crystal reveal a wealth of information on {approx} 1-10 eV electronic excitations. The IXS signal resonates when the incident photon energy approaches the pre-edge (1s{sup -}-3d) and the main-edge (1s{sup -}-4p) of the Fe K-edge absorption spectrum. The RIXS spectra measured at the pre-edge and the main-edge show quantitatively different dependences on the incident photon energy, momentum transfer, photon polarization, and temperature. We present a multielectron analysis of the Mott-Hubbard (MH) and charge transfer (CT) excitations, and calculate their energies. Electronic excitations observed in the pre-edge and main-edge RIXS spectra are interpreted as MH and CT excitations, respectively. We propose the electronic structure around the chemical potential in FeBO{sub 3} based on the experimental data.

Femto-second transient absorption spectroscopy has been applied to investigate the back-electron transfer process after the photo-excitation of p-nitroaniline (PNA) in water from ambient to supercritical condition. After PNA was photo-excited at 400 nm, the bleach recovery signal was observed due to the back-electron transfer from the excited state to the ground state within about 1 ps. In the longer wavelength region above 400 nm, a transient absorption signal was observed due to the hot-band absorption which was produced from the back-electron transfer process to the ground state. The hot band decay rate was determined along the isochoric line from 298 K to 664 K at 40.1 MPa. The hot band decay rate increases from 298 K to 473 K, then it decreases to 664 K. The rate maximum was reproduced by the density and temperature dependence of the collision frequency, although the decrease at the higher temperature region was much more moderate for the hot band decay rate than is predicted by the collision frequency, suggesting the effects of the local density enhancement and the solute-solvent hydrogen-bonding.

The electronic structure and some electron transfer properties of a model mixed-valence Spiro molecular cation have been investigated at CAS-SCF, CAS+S, and CAS+SD levels starting from canonical and localized orbitals, using SZ, DZ, and TZP basis sets. The potential energy surfaces of the adiabatic ground and the lowest three excited electronic states have been computed, within a two-state model, and a double-well potential has been obtained for the ground electronic state. We have demonstrated the low coupling interaction between the two redox moieties of this molecular cation by following the charge localization/delocalization in the valence p system through the reaction coordinate of the intramolecular charge transfer. The effect of dynamical correlation, using either localized or canon...

Recent experiments by several groups have examined the question of population transfer to resonantly excited states during intense short laser pulses, in particular the amount of population that remains ``trapped`` in excited states at the end of a laser pulse. In this chapter we present calculations of population transfer and resonant ionization in xenon at both 660 and 620 nm. At the longer wavelength, the seven photon channel closes at 2.5{times}10{sup 13} W/cm{sup 2}. Pulses with peak intensities higher than this result in ``Rydberg trapping``, the resonant transfer of population to a broad range of high-lying states. The amount of population transferred depends on both the peak intensity and pulse duration. At 620 mm there are numerous possible six photon resonances to states with p or f angular momentum. We have done a large number of calculations for 40 fs pulses at different peak intensities and have examined the population transferred to these low-lying resonant states as a function of the peak laser intensity. We do not have room to comment upon the resonantly enhanced ionized electron energy spectra that we also determine in the same calculations. Our calculations involve the direct numerical integration of the time-dependent Schroedinger equation for an atom interacting with a strong laser field. The time-dependent wave function of a given valence electron is calculated on a spatial grid using a one-electron pseudo potential. This single active electron approximation (SAE) has been shown to be a good approximation for the rare gases at the intensities and wavelengths that we will consider. The SAE potential we use has an explicit angular momentum dependence which allows us to reproduce all of the excited state energies for xenon quite well.

Carotenoids, natural pigments widely distributed in algae and plants, have a conjugated double bond system. Their excitation energies are correlated with conjugation length. We hypothesized that carotenoids whose energy states are above the singlet excited state of oxygen (singlet oxygen) would possess photosensitizing properties. Here, we demonstrated that human skin melanoma (A375) cells are damaged through the photo-excitation of several carotenoids (neoxanthin, fucoxanthin and siphonaxanthin). In contrast, photo-excitation of carotenoids that possess energy states below that of singlet oxygen, such as ?-carotene, lutein, loroxanthin and violaxanthin, did not enhance cell death. Production of reactive oxygen species (ROS) by photo-excited fucoxanthin or neoxanthin was confirmed using a reporter assay for ROS production with HeLa Hyper cells, which express a fluorescent indicator protein for intracellular ROS. Fucoxanthin and neoxanthin also showed high cellular penetration and retention. Electron spin resonance spectra using 2,2,6,6-tetramethil-4-piperidone as a singlet oxygen trapping agent demonstrated that singlet oxygen was produced via energy transfer from photo-excited fucoxanthin to oxygen molecules. These results suggest that carotenoids such as fucoxanthin, which are capable of singlet oxygen production through photo-excitation and show good penetration and retention in target cells, are useful as photosensitizers in photodynamic therapy for skin disease. PMID:22185691

{beta}-Lapachone (1), a substituted o-naphthoquinone absorbing into the visible ({lambda}{sub max} = 424 nm in benzene), is cleanly and efficiently reduced to the corresponding semiquinone radical upon photolysis in degassed solutions with alcohols, amines, and {beta}-amino alcohols. The course and products of these photoreactions have been followed by NMR, ESR, fluorescence, and absorption spectroscopy. For all three types of reductant the overall reaction involves 2e{sup {minus}} oxidation of the donor, and the quantum efficiencies show a dependence upon quinone concentration indicative of the role of a second dark reduction of 1 by products of the primary photolysis. For amines and amino alcohols the reaction is initiated by single electron transfer quenching of triplet 1. For triethylamine the mechanism is indicated to be a sequence of two electron transfer-proton transfer steps culminating in two semiquinone radicals and the enamine Et{sub 2}NCH{double bond}CH{sub 2}. For amino alcohols a C-C cleavage concurrent with deprotonation of the alcohol (oxidative photofragmentation) occurs, in competition with reverse electron transfer, following the quenching step. For both amines and amino alcohols, limiting efficiencies of reaction approach 2 (for QH{sup {sm bullet}} formation). In contrast, both 2-propanol and benzyl alcohol are oxidized by excited states of 1 with much lower efficiency. The probable mechanism for photooxidation of the alcohols involves a H atom abstraction quenching of the excited state followed by an electron transfer-proton transfer sequence in which a ground-state 1 is reduced. Lower limiting efficiencies for photoreduction of 1 by the alcohols are attributed to inefficiencies of net H-atom transfer in the quenching step. 54 refs., 3 figs., 9 tabs.

Phycobilisomes are supramolecular aggregates of phycobiliproteins which functions as the major light harvesting antennae in blue-green and red algae. Isolation of intact phycobilisomes has now been accomplished from many algae. The major criterion for the functional integrity of these organelles is the demonstration that they exhibit highly efficient transfer of excitation energy from phycoerythrin (PE) to phycocyanin (PC) and finally to allophycocyanin (APC). A structural model for the phycobilisomes of Porphyridium cruentum was first proposed in on the basis of kinetics of pigments released, concomitant energy transfer uncoupling, and electron microscopy studies. Accordingly, phycobilisomes consisted of an allophycocyanin core, surrounded by phycocyanin, and phycoerythrin (when present) on the periphery.

We study theoretically the resonant inelastic x-ray scattering in quasi-one-dimensional insulating copper oxides, where the incident photon energy is tuned to the Cu 1s–4p absorption energy. Our attention is focused particularly on the strong momentum-transfer dependence of the spectral shape observed in recent experiments. We describe the antiferromagnetic ground state within the Hartree–Fock theory, and consider charge excitations from the ground state within the random phase approximation. By taking account of the electron correlation effects perturbatively, we obtain the detailed momentum-transfer dependence of the spectra in semiquantitative agreement with the experiments.   

A series of mononuclear and dinuclear cyclometalated platinum(II) 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2'-bipyridine complexes (F-1-F-5) were synthesized and their photophysical properties were systematically investigated. All complexes exhibit strong (1)pi,pi* absorption bands in the UV region, and a broad, structureless charge transfer band in the visible region. The charge-transfer band is broadened and red-shifted for F-3-F-5 compared to those for F-1 and F-2 because of the electron-donating acetylide ligand and the involvement of the ligand-to-ligand charge transfer character. The molar extinction coefficients for the dinuclear complex F-5 are much higher than those for the mononuclear complexes F-1-F-4, indicating the electronic coupling through the bridge ligand. All complexes are emissive in solution at room temperature and in glassy matrix at 77 K. When excited at the charge transfer absorption band, the complexes exhibit a long-lived red/orange emission around 600 nm, which is attributed to a triplet metal-to-ligand charge transfer/intraligand charge transfer emission ((3)MLCT/(3)ILCT). For emission at 77 K, the emitting state is tentatively assigned as (3)MLCT for F-2-F-4, and (3)MLCT/(3)pi,pi* for F-1 and F-5 taking into account the emission energy, the shape of the spectrum, the lifetime, and the thermally induced Stokes shift. F-1-F-4 exhibit broad triplet transient difference absorption in the visible to the near-IR region, with a lifetime comparable to those measured from the decay of the (3)MLCT/(3)ILCT emission. Therefore, F-1-F-4 give rise to a strong reverse saturable absorption for ns laser pulses at 532 nm. Z-scan experiments were carried out at 532 nm using both ns and ps laser pulses, and the experimental data was fitted by a five-band model to extract the singlet and triplet excited-state absorption cross sections. The degree of reverse saturable absorption follows this trend: F-1 = F-2 > F-3 > F-4 > F-5, which is mainly determined by the ratio of the triplet excited-state absorption cross-section to that of the ground-state and the triplet excited-state quantum yield. Comparison of the photophysics of F-1, F-2, and F-3 to those of their corresponding Pt complexes without the fluorenyl substituent discovers that F-1-F-3 exhibit larger molar extinction coefficients for their low-energy charge transfer absorption band, longer triplet excited-state lifetimes, higher emission quantum yields, and increased ratios of the excited-state absorption cross-section to that of the ground-state. PMID:20405851

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 the excitation of the projectile followed by deexcitation via photon emission. RTE, which is analogous to dielectronic recombination (DR), proceeds via the inverse of an Auger transition, and is expected to be resonant for projectile velocities corresponding to the energy of the ejected electron in the Auger process. RTE was investigated by measuring cross sections for projectile K x-ray emission coincident with single electron capture for 15 to 200 MeV /sub 16/S/sup 13 +/, 100 to 360 MeV /sub 20/Ca/sup 16 +/ /sup 17 +/ /sup 18 +/ and 180 to 460 MeV /sub 23/V/sup 19 +/ /sup 20 +/ /sup 21 +/ ions colliding with helium. Strong resonant behavior, in agreement with theoretical calculations of RTE, was observed in the coincidence cross sections.

The electronic spectra of {alpha} and {beta} solid O{sub 2} were calculated in a full many-body approach for a cluster consisting of four O{sub 2} molecules with periodic boundary conditions. By including only the partially occupied {pi} orbitals (16 spin-orbitals, 8 electrons) the basis set consists of 12870 many-electron states. Use of the symmetry properties (group-theoretical analysis) simplifies the problem considerably. Resulting spectra -- with phenomenological Hamiltonian parameters obtained from experiment -- consist of separate regions: (a) 81 states corresponding to the ground state and low-energy magnetic excitations (magnons); (b) 1215 states of neutral molecular excitations (excitons); and (c) 11574 charge-transfer states (conducting high-energy states). Analysis of the properties of the ground states in both {alpha} and {beta} solid O{sub 2} has been carried out.

Sensitive interference detection of the electric field of femtosecond four-wave mixing signals (stimulated photon echoes) at their point of origin in the sample can be used to record two-dimensional (2D) Fourier transform electronic spectra. In direct analogy to 2D nuclear magnetic resonance, 2D Fourier transform spectra have nearly homogeneous linewidths in each frequency dimension and sort the signal spectrum according to the initial excitation frequency. The initial excitation frequency information is stored in a robust population grating, so 2D spectra can be used to study both coherent and incoherent processes, and have revealed coherent aspects of energy transfer processes. Femtosecond 2D spectra also have the advantage of ``freezing out'' vibrational motions as inhomogeneities, raising interesting questions about what kinds of broadening can be rephased in 2D spectra recorded with stimulated photon echo pulse sequences. This talk will focus on coherent aspects of non-adiabatic electronic curve crossing and their manifestation in 2D electronic spectra.

The nature of the initially excited state of the primary electron donor or special pair has been investigated by Stark effect spectroscopy for reaction centers from the photosynthetic bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides at 77K. The data provide values for the magnitude of the difference in permanent dipole moment between the ground and excited state, |? ?|, and the angle zeta between ? ? and the transition dipole moment for the electronic transition. |? ?| and zeta for the lowest-energy singlet electronic transition associated with the special pair primary electron donor were found to be very similar for the two species. |? ?| for this transition is substantially larger than for the Qy transitions of the monomeric pigments in the reaction center or for pure monomeric bacteriochlorophylls, for which Stark data are also reported. We conclude that the excited state of the special pair has substantial charge-transfer character, and we suggest that charge separation in bacterial photosynthesis is initiated immediately upon photoexcitation of the special pair. Data for Rhodobacter sphaeroides between 340 and 1340 nm are presented and discussed in the context of the detection of charge-transfer states by Stark effect spectroscopy.

One-dimensional (1D) correlated electron systems are good targets for the exploration of photoinduced phase transitions (PIPTs). This is because photocarrier generations and/or charge transfer (CT) excitations by lights can stimulate instabilities inherent to the 1D nature of electronic states through strong electron–electron interactions and electron(spin)–lattice interactions. In this paper, we review the ultrafast dynamics of three typical PIPTs observed in 1D correlated electron systems: 1) a photoinduced transition from a Mott insulator to a metal in a halogen-bridged Ni-chain compound, [Ni(chxn)2Br]Br2 (chxn = cyclohexanediamine); 2) a photoinduced melting of a spin-Peierls phase in an organic CT compound, K-tetracyanoquinodimethane (TCNQ); 3) a photoinduced transition between neutral (N) and ionic (I) states in an organic CT compound, tetrathiafulvalene-p-chloranil (TTF-CA). The primary dynamics of these PIPTs are discussed on the basis of the results of femtosecond pump–probe spectroscopy.   

In the two-step pumping scheme for a gamma-ray laser, an essential step is that of exciting the nucleus from a long-lived storage isomer to a nearby short-lived state that then decays to the upper lasing level. For a crystalline structure host, the radiation must be used efficiently so as not to destroy the crystal. High intensity sources of photons are available only for relatively low quantum energy, but it is difficult to couple a long-wavelength photon directly to the much smaller nucleus. An experiment is proposed to induce this transfer by first exciting the atomic electrons. The nuclear excitation should occur by the exchange of a virtual photon in the near field of the electrons. As a test case, the 73 eV /sup 235m/U isomer might be excited by electronic motions induced by a high-brightness uv laser. The conversion electrons from the decay of the isomer would be detected and a 26 minute decay curve would indicate induced nuclear transitions. 5 refs.

In this review we discuss structure-function relationships of the core complex of photosystem II, as uncovered from analysis of optical spectra of the complex and its subunits. Based on descriptions of optical difference spectra including site directed mutagenesis we propose a revision of the multimer model of the symmetrically arranged reaction center pigments, described by an asymmetric exciton Hamiltonian. Evidence is provided for the location of the triplet state, the identity of the primary electron donor, the localization of the cation and the secondary electron transfer pathway in the reaction center. We also discuss the stationary and time-dependent optical properties of the CP43 and CP47 subunits and the excitation energy transfer and trapping-by-charge-transfer kinetics in the core complex. PMID:21531572

In this review we discuss structure-function relationships of the core complex of photosystem II, as uncovered from analysis of optical spectra of the complex and its subunits. Based on descriptions of optical difference spectra including site directed mutagenesis we propose a revision of the multimer model of the symmetrically arranged reaction center pigments, described by an asymmetric exciton Hamiltonian. Evidence is provided for the location of the triplet state, the identity of the primary electron donor, the localization of the cation and the secondary electron transfer pathway in the reaction center. We also discuss the stationary and time-dependent optical properties of the CP43 and CP47 subunits and the excitation energy transfer and trapping-by-charge-transfer kinetics in the co...

A series of one-, two-, and three-branched chromophores based on 3-hydroxyflavones (1-3) have been synthesized as the first example of multibranched chromophores demonstrating excited-state intramolecular proton transfer (ESIPT). Coupling between the 3-hydroxyflavone branches connected by an electron-donating triphenylamine core is manifested in the red-shifted and asymmetric absorption band of 2, whereas the absorption of 3 is governed by the divided donor strength. Their excited-state charge-transfer (ESCT)-coupled ESIPT dynamics is investigated via femtosecond fluorescence upconversion and is proved to be well correlated with the ratio of normal/tautomer emission in the fluorescence spectra. For 1 and 2, with increased donor strength compared with the 4'-N,N-dialkylamino-3-hydroxyflavone analogue, ESIPT appears to cease in the more polar solvent of acetonitrile. Nevertheless, similar dependence of 1-3 on solvent polarity signifies resembling charge-transfer character at the normal excited states (N*), despite their varying structures. As evidenced by the theoretical approach, the frontier orbitals of vibrationally relaxed (geometry-optimized) N*, from which fluorescence and ESIPT should take place, are localized on one specific branch, leading to similar emission patterns and dynamics, whereas the orbitals contributing to Franck-Condon excitation (absorption) spread over the entire molecule. The localization is found to be facilitated by rotation of a specific branch pivoting on the central nitrogen atom, while planarity is maintained within each 3-hydroxyflavone chromophore. PMID:20822165

Differentiation of the filamentous cyanobacteria Calothrix sp strains PCC 7601 and PCC 7504 is regulated by light spectral quality. Vegetative filaments differentiate motile, gas-vacuolated hormogonia after transfer to fresh medium and incubation under red light. Hormogonia are transient and give rise to vegetative filaments, or to heterocystous filaments if fixed nitrogen is lacking. If incubated under green light after transfer to fresh medium, vegetative filaments do not differentiate hormogonia but may produce heterocysts directly, even in the presence of combined nitrogen. We used inhibitors of thylakoid electron transport (3-[3,4-dichlorophenyl]-1,1-dimethylurea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) to show that the opposing effects of red and green light on cell differentiation arise through differential excitations of photosystems I and II. Red light excitation of photosystem I oxidizes the plastoquinone pool, stimulating differentiation of hormogonia and inhibiting heterocyst differentiation. Conversely, net reduction of plastoquinone by green light excitation of photosystem II inhibits differentiation of hormogonia and stimulates heterocyst differentiation. This photoperception mechanism is distinct from the light regulation of complementary chromatic adaptation of phycobilisome constituents. Although complementary chromatic adaptation operates independently of the photocontrol of cellular differentiation, these two regulatory processes are linked, because the general expression of phycobiliprotein genes is transiently repressed during hormogonium differentiation. In addition, absorbance by phycobilisomes largely determines the light wavelengths that excite photosystem II, and thus the wavelengths that can imbalance electron transport.

Three new photoinduced electron donor-acceptor (D-A) systems are reported which juxtapose a Ru(II) excited-state donor with a bipyridinium acceptor via a conformationally active asymmetric aryl-substituted bipyridine ligand participating in the bridge between D and A. Across the series of complexes 1-3, steric bulk is sequentially added to tune the inter-ring dihedral angle theta between the bipyridine and the aryl substituent. Driving forces for photoinduced electron transfer (DeltaG(ET)) and back electron transfer (DeltaG(BET)) are reported based on electrochemical measurements of 1-3 as well as Franck-Condon analysis of emission spectra collected for three new donor model complexes 1'-3'. These preserve the substitution patterns on the aryl substituent in their respective D-A complexes but remove the bipyridinium acceptor. Both DeltaG(ET) and DeltaG(BET) are invariant to within 0.02 eV across the series. Upon visible photoexcitation of each of the D-A systems with approximately 100 fs laser pulses at 500 +/- 10 nm, an electron-transfer (ET) photoproduct is observed to form with a time constant of tau(ET) = 29 ps (1), 37 ps (2), and 57 ps (3). That ET remains relatively rapid throughout this series, even as steric bulk significantly increases the inter-ring dihedral angle theta, is attributed to the effects of ligand-based torsional dynamics driven by intraligand electron delocalization in the D*-A excited state manifold prior to ET. The lifetimes of the charge-separated states (tau(BET)) are also reported with tau(BET) = 98 ps (1), 217 ps (2), and 789 ps (3), representing a more than 8-fold increase across the series. This is attributed to reverse conformational dynamics in D(+)-A(-) driven by steric repulsions, which serves to minimize electronic coupling to the ground state. Steric control of ligand geometry and the range over which theta changes during conformational dynamics provides a new strategy to facilitate the formation and storage of charge-separated excited states. PMID:20684515

Quantum-yield and thermodynamic analyses of the triplet-sensitized photolysis of the title hydroxylamine (NT, 1) with 4,4?-bis(dimethylamino)benzophenone (BAB), 4-dimethylaminobenzophenone (DAB), 4,4?-dimethoxybenzophenone (DMB), and 3,3?-bis(trifluoromethyl)benzophenone (BFB) suggest that the BAB- and DAB-sensitized reactions proceed by a preferential electron-transfer pathway within a triplet-exciplex intermediate, formed between the triplet-state sensitizer and the ground-state NT, to give N-phenyl-1-naphthalenecarboxamide (2) and p-toluic acid (3). On the other hand, the energy-transfer pathway in this exciplex intermediate, eventually forming 2 and 3, as well as toluene (4), predominates for reactions sensitized with DMB and BFB having fewer electron-donating substituents than does the dimethylamino group of BAB and DAB. The finding that benzil, both excited singlet and triplet states of which lie below those of NT, brings about a sensitized photolysis to afford the same fragmentation products, provides strong evidence for the existence of a triplet-exciplex intermediate. In addition, evidence concerning the operation of an electron-transfer mechanism in our sensitized photolysis comes from an observation that the photolysis of NT sensitized with N,N,N?,N?-tetramethylbenzidine, the excited state of which is one of the most powerful one-electron reductants, gives the same product distribution as does that obtained by the BAB- and DAB-sensitized reactions.   

Photosystem II (PSII) is a multisubunit protein complex in cyanobacteria, algae and plants that use light energy for oxidation of water and reduction of plastoquinone. The conversion of excitation energy absorbed by chlorophylls into the energy of separated charges and subsequent water-plastoquinone oxidoreductase activity are inadvertently coupled with the formation of reactive oxygen species (ROS). Singlet oxygen is generated by the excitation energy transfer from triplet chlorophyll formed by the intersystem crossing from singlet chlorophyll and the charge recombination of separated charges in the PSII antenna complex and reaction center of PSII, respectively. Apart to the energy transfer, the electron transport associated with the reduction of plastoquinone and the oxidation of water is linked to the formation of superoxide anion radical, hydrogen peroxide and hydroxyl radical. To protect PSII pigments, proteins and lipids against the oxidative damage, PSII evolved a highly efficient antioxidant defense system comprising either a non-enzymatic (prenyllipids such as carotenoids and prenylquinols) or an enzymatic (superoxide dismutase and catalase) scavengers. It is pointed out here that both the formation and the scavenging of ROS are controlled by the energy level and the redox potential of the excitation energy transfer and the electron transport carries, respectively. The review is focused on the mechanistic aspects of ROS production and scavenging by PSII. This article is part of a Special Issue entitled: Photosystem II. PMID:21641332

Nonlinear response of an asymmetric GaAs/AlGaAs triple quantum well structure (ATQW) is studied with femto-second pump-probe transmission at 300 K. The strong optical nonlinear behavior is obtained in this system by He-Ne laser for the first time. The nonlinearity is due to the quantum confined Stark effect induced by the built-in field. The time constant of the transmission change after the input of the pump pulse is found 12 {approx} 20 ps, which is attributed to the tunneling transfer of photo-excited electrons near the triple resonance of the electronic states.

Graphene-CdS (G-CdS) composites were synthesized through a simple solvothermal method. The formed CdS nanospheres were homogeneously scattered on the surface of graphene sheets. Fluorescence quenching effect of the G-CdS composites indicated effective transfer of photo-excited electrons from CdS to graphene, suppressed the recombination of photo-generated electron-hole pairs, so that the enhanced visible light induced photodegradation activity for Rhodamine B (RhB) was achieved. Based on the high photocatalytic activity and well stability, the G-CdS composite containing 70% CdS can be expected to be a practical visible light photocatalyst.

Using thin film pillars ~100 nm in diameter, containing two ferromagnetic Co layers of different thicknesses separated by a paramagnetic Cu spacer, we examine effects of torques due to spin-polarized currents flowing perpendicular to the layers. In accordance with spin-transfer theory, spin-polarized electrons flowing from the thin to the thick Co layer can switch the magnetic moments of the layers antiparallel, while a reversed electron flow causes switching to a parallel state. When large magnetic fields are applied, the current no longer fully reverses the magnetic moment, but instead stimulates spin-wave excitations.

We researched contribution of a charge transfer (CT) effect to desorption/ionization mechanism in our ultra-high sensitive laser desorption/ionization mass spectrometry based on surface plasmon (SP) excitation (SPLDI-MS). A quantity of CT electrons between a metal surface and sample molecules estimated from Raman measurement is correlated to a mass signal intensity of sample molecule in our SPLDI-MS method. A sample system with a larger quantity of CT electrons gave a higher mass signal intensity. Efficient use of the CT effect would lead to development of a higher sensitive SPLDI-MS and this development would contribute to advancement of various fields. [DOI: 10.1380/ejssnt.2009.93]   

This project seeks to understand the mechanism of elementary chemical reactions on solid surfaces, with implications for heterogeneous catalysis, atmospheric chemistry, and materials synthesis. The emphasis of the study is placed on an atomic scale understanding by probing the fundamental motions of individual molecules such as vibration, rotation, conformational change, and nuclear motions leading to bond dissociation and formation. The pathways of energy flow during a chemical transformation are investigated by considering the excitation source (photons, tunneling electrons, and heat or phonons), electronic and nuclear relaxation mechanisms and rates, and charge and energy transfers.

A series of complex salts in which trans-bis[1,2-phenylenebis(dimethylarsine)]chlororuthenium(II) electron donor groups are connected to pyridyl or pyridinium electron acceptors has been prepared. These chromophores exhibit intense, visible metal-to-ligand charge-transfer (MLCT) absorptions and reversible Ru(III/II) (and also in some cases ligand-based) redox processes. Stark (electroabsorption) spectroscopic studies have been used to determine dipole moment changes for the MLCT excitations. Static first hyperpolarizabilities have been calculated according to the two-state model, allowing the derivation of structure-activity correlations for the molecular quadratic nonlinear optical responses.

1-[(Anthracen-9-yl)methylene] thiosemicarbazide shows weak fluorescence due to a photo-induced electron transfer (PET) process from the thiosemicarbazide moiety to the excited anthracene. The anthracene emission can be recovered via protonation of the amine as the protonated aminomethylene as an electron-withdrawing group that suppresses the PET process. Similarly, chelation between the ligand and the metal ions can also suppress the PET process and results in a fluorescence enhancement (CHEF). When solvents are introduced as the third control, a molecular 2:1 multiplexer is constructed to report selectively the inputs. Therefore, a molecular 2:1 multiplexer is realized in a simple molecular system. PMID:22666037

In this work, we introduce a new modified approach to the formation of interdigital transducer (IDT) structures on an AlGaN/GaN heterostructure. The approach is based on a shallow recess-gate plasma etching of the AlGaN barrier layer in combination with "in-situ" SF6 surface plasma treatment applied selectively under the Schottky gate fingers of IDTs. It enables one to modify the two-dimensional electron gas (2DEG) density and the surface field distribution in the region of the IDTs, as is needed for the excitation of a surface acoustic wave (SAW). The measured transfer characteristics of the plasma-treated SAW structures revealed the excitation of SAW at zero bias voltage due to fully depleted 2DEG in the region of the IDTs. High external bias voltages are not necessary for SAW excitation...