Showing papers on "Excited state published in 2002"
TL;DR: In this paper, the authors present theory, implementation, and validation of excited state properties obtained from time-dependent density functional theory (TDDFT), based on a fully variational expression for the excited state energy, a compact derivation of first order properties is given.
Abstract: This work presents theory, implementation, and validation of excited state properties obtained from time-dependent density functional theory (TDDFT). Based on a fully variational expression for the excited state energy, a compact derivation of first order properties is given. We report an implementation of analytic excited state gradients and charge moments for local, gradient corrected, and hybrid functionals, as well as for the configuration interaction singles (CIS) and time-dependent Hartree–Fock (TDHF) methods. By exploiting analogies to ground state energy and gradient calculations, efficient techniques can be transferred to excited state methods. Benchmark results demonstrate that, for low-lying excited states, geometry optimizations are not substantially more expensive than for the ground state, independent of the molecular size. We assess the quality of calculated adiabatic excitation energies, structures, dipole moments, and vibrational frequencies by comparison with accurate experimental data for a variety of excited states and molecules. Similar trends are observed for adiabatic excitation energies as for vertical ones. TDDFT is more robust than CIS and TDHF, in particular, for geometries differing significantly from the ground state minimum. The TDDFT excited state structures, dipole moments, and vibrational frequencies are of a remarkably high quality, which is comparable to that obtained in ground state density functional calculations. Thus, yielding considerably more accurate results at similar computational cost, TDDFT rivals CIS as a standard method for calculating excited state properties in larger molecules.
1,976 citations
TL;DR: In this paper, the applicability of the well-known Judd-Ofelt theory to the emissive properties of Eu3+ complexes is investigated, and it is demonstrated experimentally that the radiative lifetime of the 5D0 excited state of the Eu 3+ can be calculated directly from its corrected emission spectrum, without using Judd-ofelt theory.
Abstract: Although luminescent complexes of lanthanide ions and organic ligands have been studied intensively, relatively little attention has been paid to the natural (or ‘radiative’) lifetime of the lanthanide centered luminescent state in these systems. Here, the applicability of the well-known Judd–Ofelt theory to the emissive properties of Eu3+ complexes is investigated. Moreover, it is demonstrated experimentally that the radiative lifetime of the 5D0 excited state of Eu3+ can be calculated directly from its corrected emission spectrum, without using Judd–Ofelt theory. We also discuss briefly the possibility of finding the natural lifetimes of lanthanide ions other than Eu3+.
1,142 citations
TL;DR: In this article, the authors combined results of ab initio electronic-structure calculations and spectroscopic investigations of jet-cooled molecules and clusters provide strong evidence of a surprisingly simple and general mechanistic picture of the nonradiative decay of biomolecules such as nucleic bases and aromatic amino acids.
Abstract: The combined results of ab initio electronic-structure calculations and spectroscopic investigations of jet-cooled molecules and clusters provide strong evidence of a surprisingly simple and general mechanistic picture of the nonradiative decay of biomolecules such as nucleic bases and aromatic amino acids. The key role in this picture is played by excited singlet states of πσ* character, which have repulsive potential-energy functions with respect to the stretching of OH or NH bonds. The 1πσ* potential-energy functions intersect not only the bound potential-energy functions of the 1ππ* excited states, but also that of the electronic ground state. Via predissociation of the 1ππ* states and a conical intersection with the ground state, the 1πσ* states trigger an ultrafast internal-conversion process, which is essential for the photostability of biomolecules.
In protic solvents, the 1πσ* states promote a hydrogen-transfer process from the chromophore to the solvent. Calculations for chromophore–water clusters have shown that a spontaneous charge-separation process takes place in the solvent shell, yielding a microsolvated hydronium cation and a microsolvated electron. These results suggest that the basic mechanisms of the complex photochemistry of biomolecules in liquid water can be revealed by experimental and theoretical investigations of relatively small chromophore–water clusters.
866 citations
TL;DR: The present work focuses on the applications of magnetic Resonance Spectroscopy in the area of Aqueous Media Research, where Lanthanide-Induced Shifts and Responsive Luminescent Systems are concerned.
Abstract: D. Intermolecular Ligand Exchange 1996 1. Water Substitution 1996 2. Non-Covalent Binding 1997 V. Excited-State Chemistry 1998 A. Excitation and Quenching 1998 B. Emission Characteristics 1999 C. Responsive Luminescent Systems 2002 VI. Magnetic Resonance Spectroscopy Applications 2004 A. Lanthanide-Induced Shifts (LIS) 2004 B. Applications in Aqueous Media 2005 VII. Acknowledgements 2007 VIII. Supporting Information 2007 IX. References 2007
796 citations
TL;DR: More precise values have been obtained for two previously proposed absolute quantum yield standards for the rhodamine 6G cation and the fluorescein dianion dyes in nine solvents.
Abstract: Absolute fluorescence quantum yields are reported for the rhodamine 6G cation and the fluorescein dianion dyes in nine solvents. This information is combined with previously reported fluorescence lifetimes to deduce radiative and nonradiative decay rates. Along the alcohol series from methanol to octanol, rhodamine 6G displays an increasing radiative rate, in parallel with the square of the refractive index increase, and a slightly decreasing nonradiative rate. Fluorescein is different: the apparent radiative rate actually decreases, suggesting that the emissive species is perturbed in some fashion. For both dyes, fluorescence yields are enhanced in D2O, rising to 0.98, in parallel with a corresponding increase in lifetimes. Protonated solvents invariably give shorter lifetimes and lower quantum yields, contrary to some previous speculation. From this work and an analysis of existing literature values, more precise values have been obtained for two previously proposed absolute quantum yield standards. The yield of fluorescein in 0.1 N NaOH(aq) is 0.925+/-0.015, and for rhodamine 6G in ethanol, it is 0.950+/-0.015. In both cases, the solutions are assumed to be in the limit of low concentration, excited close to their long-wave absorption band and at room temperature but may be either air-saturated or free of oxygen.
719 citations
TL;DR: In this article, the ground state and low-lying excited electronic states in the Ir(III) complex Ir(ppy)3, and in the related complexes Ir (ppy)2(acac) and Ir(ppy)-2(bza), are studied using density functional theory techniques.
Abstract: The ground state and low-lying excited electronic states in the Ir(III) complex Ir(ppy)3, and in the related complexes Ir(ppy)2(acac) and Ir(ppy)2(bza), are studied using density functional theory techniques [where ppy = 2-phenylpyridine, acac = acetoylacetonate, and bza = benzyolacetonate]. Ir complexes of ppy have been the subject of numerous photophysical absorption and luminescence experiments and have been examined as potential donors in organic light emitting diodes (OLEDs). The electronic properties of the neutral molecules, in addition to the positive and negative ions, are studied using the B3LYP functional. Optimized geometries are compared to experimentally observed structures. Excited triplet and singlet states are examined using time-dependent density functional theory (TDDFT). The calculated energies of the lowest triplet state (2.4−2.6 eV) and lowest singlet state (2.6−2.7 eV) in the three complexes are in good agreement with experimental absorption spectra and luminescence studies. All of ...
682 citations
TL;DR: The enhanced excited state decay process for NCs coupled to rough metal substrates effectively competes with the Auger relaxation process, allowing us to observe both charged and neutral exciton emission from these NC quantum dots.
Abstract: The fluorescence behavior of single CdSe(ZnS) core-shell nanocrystal (NC) quantum dots is dramatically affected by electromagnetic interactions with a rough metal film. Observed changes include a fivefold increase in the observed fluorescence intensity of single NCs, a striking reduction in their fluorescence blinking behavior, complete conversion of the emission polarization to linear, and single NC exciton lifetimes that are >10(3) times faster. The enhanced excited state decay process for NCs coupled to rough metal substrates effectively competes with the Auger relaxation process, allowing us to observe both charged and neutral exciton emission from these NC quantum dots.
582 citations
TL;DR: New, readily accessible copper(I) complexes that can exhibit unusually long-lived, high quantum yield emissions in fluid solution are described.
Abstract: This report describes new, readily accessible copper(I) complexes that can exhibit unusually long-lived, high quantum yield emissions in fluid solution. The complexes are of the form [Cu(NN)(POP)]+ where NN denotes 1,10-phenanthroline (phen), 2,9-dimethyl-1,10-phenanthroline (dmp) or 2,9-di-n-butyl-1,10-phenanthroline (dbp) and POP denotes bis[2-(diphenylphosphino)phenyl] ether. Modes of characterization include X-ray crystallography and cyclic voltammetry. The complexes each have a pseudotetrahedral coordination geometry and a Cu(II)/Cu(I) potential upward of +1.2 V vs Ag/AgCl. In room-temperature dichloromethane solution, charge-transfer excited states of the dmp and dbp derivatives exhibit respective emission quantum yields of 0.15 and 0.16 and corresponding excited-state lifetimes of 14.3 and 16.1 μs, respectively. Despite the fact that coordinating solvents usually quench charge-transfer emission from copper systems, the photoexcited dmp (dbp) complex retains a lifetime of 2.4 μs (5.4 μs) in methanol.
576 citations
TL;DR: In this article, the luminescence properties of chemically prepared gold nanoclusters, each composed of a 28-atom core and a glutathione (GSH) adsorbate layer consisting of 16 molecules, were investigated.
Abstract: The luminescence properties of chemically prepared gold nanoclusters, each composed of a 28-atom core and a glutathione (GSH) adsorbate layer consisting of 16 molecules, were investigated These clusters show a distinct absorption onset at 13 eV corresponding to the opening of an electronic gap within the conduction band (HOMO-LUMO gap) Here we report on the radiative properties of these molecular-like gold clusters By using a combination of different detectors with sensitivities in the visible to the infrared (20−08 eV), a broad luminescence extending over this entire spectral range was observed Our results further suggest that the luminescence can be separated into two bands with maxima around 15 and 115 eV indicating that radiative recombination between the ground state and two distinctively different excited states takes place The origin of the observed luminescence bands is discussed using a solid state as well as a molecular model for the electronic structure and relaxation of the clusters
546 citations
TL;DR: In this paper, a triple-quadrupole double-octopole (TQDO) photoionization mass spectrometer has been developed for total cross section measurements of state-selected ion−molecule reactions.
Abstract: A unique triple-quadrupole double-octopole (TQDO) photoionization mass spectrometer has been developed for total cross section measurements of state-selected ion−molecule reactions. By employing this TQDO apparatus, we have recently examined the absolute total cross sections for a series of state-selected ion−molecule reactions involving Ar+(2P3/2,1/2), O+(4S, 2D, 2P), and organosulfur ions (CH3SH+, CH3CH2SH+, and CH3SCH3+) in their ground states. The cross section measurements, together with product ion kinetic energy analyses, have provided convincing evidence that the Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) reactions proceed via a charge-transfer predissociation mechanism. The comparison of absolute cross sections for product ions formed in the dissociative charge transfer of Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) and those produced in photoionization of CO2 (CO, N2, O2) suggests that product ions formed by dissociative charge transfer are also produced by photoionization via a similar set of excited predissociat...
406 citations
TL;DR: The calculations indicate a two-step mechanism for intrachain energy transfer with hopping along the conjugated chains as the rate-limiting step; the higher efficiency of the interchain transfer process is mainly due to larger electronic coupling matrix elements between closely lying chains.
Abstract: The energy-transfer processes taking place in conjugated polymers are investigated by means of ultrafast spectroscopy and correlated quantum-chemical calculations applied to polyindenofluorenes end-capped with a perylene derivative. Comparison between the time-integrated luminescence and transient absorption spectra measured in solution and in films allows disentangling of the contributions arising from intrachain and from interchain energy-migration phenomena. Intrachain processes dominate in solution where photoexcitation of the polyindenofluorene units induces a rather slow energy transfer to the perylene end moieties. In films, close contacts between chains favors interchain transport of the excited singlet species (from the conjugated bridge of one chain to the perylene unit of a neighboring one); this process is characterized by a 1-order-of-magnitude increase in transfer rate with respect to solution. This description is supported fully by the results of quantum-chemical calculations that go beyond the usual point-dipole model approximation and account for geometric relaxation phenomena in the excited state before energy migration. The calculations indicate a two-step mechanism for intrachain energy transfer with hopping along the conjugated chains as the rate-limiting step; the higher efficiency of the interchain transfer process is mainly due to larger electronic coupling matrix elements between closely lying chains.
TL;DR: This analysis shows self-trapping of excitations on about six repeat units in the course of photoexcitation relaxation, identifies specific slow and fast nuclear motions strongly coupled to the electronic degrees of freedom, and predicts spectroscopic signatures of molecular conformations.
Abstract: Time-dependent photoexcitation and optical spectroscopy of $\ensuremath{\pi}$-conjugated molecules is described using a new method for the simulation of excited state molecular dynamics in extended molecular systems with sizes up to hundreds of atoms. Applications are made to poly(p-phenylene vinylene) oligomers. Our analysis shows self-trapping of excitations on about six repeat units in the course of photoexcitation relaxation, identifies specific slow (torsion) and fast (bond-stretch) nuclear motions strongly coupled to the electronic degrees of freedom, and predicts spectroscopic signatures of molecular conformations.
TL;DR: The results on the relaxation dynamics of the higher excited states in [Ru(bpy)(3)](2+) are valuable in explaining the role of nonequilibrated higher excited sensitizer states of transition metal complexes in the electron injection and other ultrafast processes.
Abstract: The excited-state dynamics of a transition metal complex, tris(2,2‘-bipyridine)ruthenium(II), [Ru(bpy)3]2+, has been investigated using femtosecond fluorescence upconversion spectroscopy. The relaxation dynamics in these molecules is of great importance in understanding the various ultrafast processes related to interfacial electron transfer, especially in semiconductor nanoparticles. Despite several experimental and theoretical efforts, direct observation of a Franck−Condon singlet excited state in this molecule was missing. In this study, emission from the Franck−Condon excited singlet state of [Ru(bpy)3]2+ has been observed for the first time, and its lifetime has been estimated to be 40 ± 15 fs. Biexponential decays with a fast rise component observed at longer wavelengths indicated the existence of more than one emitting state in the system. From a detailed data analysis, it has been proposed that, on excitation at 410 nm, crossover from higher excited 1(MLCT) states to the vibrationally hot triplet ...
TL;DR: In this article, the authors investigate the ionization of hydrogen in a dynamic solar atmosphere and show that the timescale for ionization/recombination is dominated by the slow collisional leakage from the ground state to the first excited state.
Abstract: We investigate the ionization of hydrogen in a dynamic solar atmosphere. The simulations include a detailed non-LTE treatment of hydrogen, calcium, and helium but lack other important elements. Furthermore, the omission of magnetic fields and the one-dimensional approach make the modeling unrealistic in the upper chromosphere and higher. We discuss these limitations and show that the main results remain valid for any reasonable chromospheric conditions. As in the static case, we find that the ionization of hydrogen in the chromosphere is dominated by collisional excitation in the Lyα transition followed by photoionization by Balmer continuum photons—the Lyman continuum does not play any significant role. In the transition region, collisional ionization from the ground state becomes the primary process. We show that the timescale for ionization/recombination can be estimated from the eigenvalues of a modified rate matrix where the optically thick Lyman transitions that are in detailed balance have been excluded. We find that the timescale for ionization/recombination is dominated by the slow collisional leakage from the ground state to the first excited state. Throughout the chromosphere the timescale is long (103-105 s), except in shocks where the increased temperature and density shorten the timescale for ionization/recombination, especially in the upper chromosphere. Because the relaxation timescale is much longer than dynamic timescales, hydrogen ionization does not have time to reach its equilibrium value and its fluctuations are much smaller than the variation of its statistical equilibrium value appropriate for the instantaneous conditions. Because the ionization and recombination rates increase with increasing temperature and density, ionization in shocks is more rapid than recombination behind them. Therefore, the ionization state tends to represent the higher temperature of the shocks, and the mean electron density is up to a factor of 6 higher than the electron density calculated in statistical equilibrium from the mean atmosphere. The simulations show that a static picture and a dynamic picture of the chromosphere are fundamentally different and that time variations are crucial for our understanding of the chromosphere itself and the spectral features formed there.
TL;DR: Pecourt et al. as discussed by the authors presented a theoretical study of isolated cytosine, the chromophore of cytidine, and showed that there must be an ultrafast decay channel for this species.
Abstract: Singlet fluorescence lifetimes of adenosine, cytidine, guanosine, and thymidine, determined by femtosecond pump−probe spectroscopy (Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2000, 122, 9348. Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2001, 123, 10370), show that the excited states produced by 263 nm light in these nucleosides decay in the subpicosecond range (290−720 fs). Ultrafast radiationless decay to the ground state greatly reduces the probability of photochemical damage. In this work we present a theoretical study of isolated cytosine, the chromophore of cytidine. The experimental lifetime of 720 fs indicates that there must be an ultrafast decay channel for this species. We have documented the possible decay channels and approximate energetics, using a valence-bond derived analysis to rationalize the structural details of the paths. The mechanism favored by our calculations and the experimental data involves (1) a two-mode decay coordinate composed of initial bond len...
TL;DR: In this article, a comprehensive treatment of the electronic excitation spectra of Mg, Zn and Ni complexes of porphyrin and porphyrazine using time-dependent density functional theory (TDDFT) is given.
TL;DR: The most stringent experimental limits or positive results known on half-life for 2β transitions to ground and excited states of daughter nuclei for different channels (2β−, 2β+, ϵβ+; 2ϵ) and modes (0ν, 2ν; 0νM; etc.) of decay are presented in this article.
TL;DR: In this paper, an inner-valence ionized cluster, which releases its excess energy by emitting an electron, is characterized by an efficient energy transfer between monomers in the cluster.
TL;DR: Evidence is presented for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1'-yl)-perylene-3,4:9,10-bis (dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively.
Abstract: Photoexcitation of chromophoric dimers constrained to a symmetric π-stacked geometry by their molecular structure usually produces excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular π systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1‘-yl)-perylene-3,4:9,10-bis(dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PDI2, was synthesized by attaching two 5PDI chromophores via imide groups to a xanthene spacer. For comparison, a linear symmetric dimer, lin-5PDI2, was prepared in which the 5PDI chromophores are linked end-to-end via a N−N single bond between their imides. The edge-to-edge π systems of the 5PDI chromophores within lin-5PDI2 are perpendicular to one another. Ground-state absorption spectra of bo...
TL;DR: In this article, the initial electronic and vibrational relaxation of the S_1 and S_2 excited states, in a system in which interference from solvent rearrangement is insignificant as evidenced by the small Stokes shift in the fluorescence.
Abstract: Femtosecond spectroscopic studies of zinc tetraphenylporphyrin (ZnTPP) in benzene and dichloromethane are reported, combining both fluorescence up-conversion and transient absorption measurements. The purpose is to investigate the initial electronic and vibrational relaxation of the S_1 and S_2 excited states, in a system in which interference from solvent rearrangement is insignificant as evidenced by the small Stokes shift in the fluorescence. Excitation of the low-lying singlet excited state (S_1) results in nanosecond relaxation, while excitation to S_2, the Soret band, leads to multiple electronic and vibrational relaxation time scales of S_2 and S_1 populations, from hundreds of femtoseconds to tens of picoseconds. The systematic and detailed studies reported here reveal that the Soret fluorescence band decays with a lifetime in benzene of 1.45 ps for excitation at 397 nm, while emission monitored at the same wavelength, but for two-photon 550 nm excitation, decays biexponentially with 200 fs and 1.0 ps time constants. In addition, the Soret fluorescence decay lifetime for 397 nm excitation is distinctly longer than the rise time of S_1 fluorescence for the same excitation, which varies with wavelength. These observations are consistent with the model presented here in which the Soret band structure consists of absorption from S_0 to two manifolds of states with distinct electronic and vibrational couplings to S_1 and higher electronic states. To compare with literature, we also measured the S_2 lifetime in dichloromethane and found it to be 1.9 ps, a lengthening from its value in benzene. However, the transient fluorescence intensity is greatly reduced. These observations in dichloromethane provide evidence of an ultrafast (<100 fs) channel for electron transfer from ZnTPP to dichloromethane for a subset of excited molecules in favorably oriented contact with the solvent, that is, a bifurcation of population. Finally, solvent-induced vibrational relaxation of the S_1 population following internal conversion from S_2 occurs over a range of time scales (picoseconds to tens of picoseconds) depending on the wavelength (fluorescence or transient absorption), and the observed rate indeed changes with solvent.
TL;DR: This work presents the first direct evidence of the presence of an intermediate singlet excited state (Sx) mediating the internal conversion from S2 to S1in carotenoids in the primary step of photosynthesis.
Abstract: We present the first direct evidence of the presence of an intermediate singlet excited state (Sx) mediating the internal conversion from S2 to S1 in carotenoids. The S2 to Sx transition is extremely fast and is completed within approximately 50 femtoseconds. These results require a reassessment of the energy transfer pathways from carotenoids to chlorophylls in the primary step of photosynthesis.
TL;DR: In this article, the femtosecond and picosecond time scale electron injection from the excited singlet and triplet states of Ru(dcbpY)(2)(NCS)(2) (RuN3) into titanium dioxide (TiO2) nanocrystalline particle film in acetonitrile was studied.
Abstract: Time-resolved absorption spectroscopy was used to study the femtosecond and picosecond time scale electron injection from the excited singlet and triplet states of Ru(dcbpY)(2)(NCS)(2) (RuN3) into titanium dioxide (TiO2) nanocrystalline particle film in acetonitrile. The fastest resolved time constant of similar to30 fs was shown to reflect a sum of two parallel ultrafast processes, nonergodic electron transfer (ET) from the initially excited singlet state of RuN3 to the conduction band of TiO2 and intersystem crossing (ISC). The branching ratio of 1.5 between the two competing processes gives rate constants of 1/50 fs(-1) for ET and 1/75 fs(-1) for ISC. Following the ultrafast processes, a minor part of the electron injection (40%) occurs from the thermalized triplet state of RuN3 on the picosecond time scale. The kinetics of this slower phase of electron injection is nonexponential and can be fitted with time constants ranging from similar to1 to similar to60 ps.
TL;DR: In this paper, a model for describing the elementary intramolecular processes in the Soret, Q_y, and Q_x electronic manifolds, with the following order of time scales and couplings, was provided.
Abstract: With femtosecond resolution, using fluorescence up-conversion and transient absorption, we have carried out measurements on free base tetraphenylporphyrin (H_2TPP) in benzene solution, pumping with ∼1300 cm^(-1) of excess vibrational energy in each of the Soret, Q_y, and Q_x bands, and also pumping the lowest vibrational band of Q_y. From these studies, made for different excitations and at different detection wavelengths, we provide a model for describing the elementary intramolecular processes in the Soret, Q_y, and Q_x electronic manifolds, with the following order of time scales and couplings: electronic (femtosecond), vibrational (femtosecond−picosecond), and singlet−triplet (nanosecond). These dynamical electronic and vibrational relaxation pathways in a molecule with small dipole in nonpolar solvents can be studied without interference from solvent reorganization, as indicated by the small Stokes shift of fluorescence. Vibrationally excited Soret → {Q_y,Q_x} and Q_y → Q_x electronic relaxation occurs in less than 100 fs, within our resolution, as evidenced by the immediate rise of Q_x fluorescence after Soret (397 nm) and Q_y (514 and 550 nm) excitation. There are generally three distinguishable ultrafast relaxation time scales within the Q_x state, which are assigned to intra- and intermolecular vibrational relaxation processes leading to thermal equilibrium in Q_x, the lowest excited singlet state. The measured time scales are as follows: 100−200 fs for intramolecular vibrational energy redistribution, 1.4 ps for vibrational redistribution caused by elastic collision with solvent molecules, and 10−20 ps for thermal equilibration by energy exchange with the solvent. Decay of the equilibrated Q_x population occurs on the nanosecond time scale by intersystem crossing to the triplet state.
TL;DR: Since the computational cost scales linearly with the number of excited states, the technique makes possible nonadiabatic ab initio simulations of systems of similar complexity to those typically studied by standard CP methods, making it ideally suited to study the photochemistry of large molecules, particularly in condensed phases.
Abstract: An extension of Car-Parrinello (CP) molecular dynamics for efficient treatment of electronically nonadiabatic processes is presented. The current approach couples the ${S}_{1}$ restricted open-shell Kohn-Sham excited state to the ${S}_{0}$ ground state using a surface hopping scheme. Efficient evaluation of the nonadiabatic couplings is achieved by exploiting the available wave function time derivatives. Since the computational cost scales linearly with the number of excited states, the technique makes possible nonadiabatic ab initio simulations of systems of similar complexity to those typically studied by standard CP methods. It is thus ideally suited to study the photochemistry of large molecules, particularly in condensed phases.
TL;DR: In this paper, the first photon echo peak shift study of liquid water is presented, where spectral diffusion within the OH stretching absorption band of HDO in D2O takes place on many time scales with a slow component on the order of 5−15 ps.
Abstract: The first photon echo peak shift study of liquid water is presented. Spectral diffusion within the OH stretching absorption band of HDO in D2O takes place on many time scales with a slow component on the order of 5−15 ps. This indicates that fluctuations of local structure of the hydrogen bond network in liquid water are relatively long-lived. Vibrational relaxation of the excited state populates a state which is spectroscopically different from the initial ground state. This leads to an strong enhancement of the peak shift and allows spectral diffusion to be measured for delay times beyond the limit determined by the population relaxation time T1. The observed signals are discussed with the help of model calculations.
TL;DR: In this article, the authors investigated individual paramagnetic defect centers in diamond nanocrystals by low-temperature high-resolution optical spectroscopy and found narrow fluorescence excitation spectral lines indicating transitions between individual spin sublevels.
Abstract: Individual paramagnetic defect centers in diamond nanocrystals have been investigated by low-temperature high-resolution optical spectroscopy. Narrow fluorescence excitation spectral lines have been found, indicating transitions between individual spin sublevels. Spectral diffusion is explained by cross relaxation among spin sublevels and by the presence of excited electrons in the conduction band of diamond. The relaxation times are in the millisecond range. The system may be useful for quantum information processing with individual electron spins.
TL;DR: The position of the Mn3+/2+ acceptor level at 1.8 eV above the valence-band edge of GaN makes the realization of carrier-mediated ferromagnetism rather unlikely in GaN:Mn as discussed by the authors.
Abstract: Molecular-beam-epitaxy grown GaN:Mn and AlN:Mn layers with Mn concentrations around 1020 cm−3 were investigated by optical absorption and photoconductivity measurements. From electron spin resonance Mn is known to be mostly present in the neutral acceptor state in GaN without codoping. This leads to a reassignment of the optical absorption features to a charge transfer from the neutral Mn3+ oxidation state, either by direct photoionization at 1.8 eV or through a photothermal ionization process via an excited state at 1.42 eV above the Mn3+ ground state by spin-allowed Mn3+ 5E→5T internal absorption. The position of the Mn3+/2+ acceptor level at 1.8 eV above the valence-band edge of GaN makes the realization of carrier-mediated ferromagnetism rather unlikely in GaN:Mn.
TL;DR: The ground and excited-state electronic structures of the photosensitizer bis(4,4‘)-dicarboxylato-2,2‘-bipyridine)-bis(isothiocyanato)ruthenium(II), the authors have been examined computationally in an effort to better understand this molecule's effectiveness in TiO2-based photoelectrochemical cells.
Abstract: The ground- and excited-state electronic structures of the photosensitizer bis(4,4‘-dicarboxylato-2,2‘-bipyridine)-bis(isothiocyanato)ruthenium(II), [RuL‘2(NCS)2]4- (where L‘ = 4,4‘-dicarboxylato-2,2‘-bipyridine), have been examined computationally in an effort to better understand this molecule's effectiveness in TiO2-based photoelectrochemical cells. Density functional theory (DFT) calculations of the compound's ground state indicate that occupied molecular orbitals (MOs) localized on carboxylate groups of the bipyridyl ligands (through which the compound binds to the TiO2 nanoparticles) energetically match the semiconductor valence band; the lowest unoccupied MOs lie above the conduction band edge and are bipyridine π* in character. These results suggest that the compound is well-positioned to bind strongly to TiO2 and engage in electron transfer from excited states associated with the bipyridyl groups. Various excited states of the chromophore were identified using time-dependent density functional th...
TL;DR: In this article, the potential energy function of the lowest 3s Rydberg state of 9H-adenine was found to be dissociative with respect to the stretching of the NH bond length of the azine group.
Abstract: Minimum-energy reaction paths and corresponding potential-energy proles have been com- puted for the lowest excited states of the amino form of 9H-adenine. Complete-active- space self-consistent- eld (CASSCF) and density functional theory (DFT) methods have been employed. The potential-energy function of the lowest 1 state, nominally a 3s Rydberg state, is found to be dissociative with respect to the stretching of the NH bond length of the azine group. The 1 potential-energy function inter- sects not only those of the 1 and 1 n excited states, but also that of the electronic ground state. The 1 { 1 and 1 {S0 intersections are converted into conical intersections when the out-of-plane motion of the active hydrogen atom is taken into account. It is argued that the predissociation of the 1 and 1 n states by the 1 state and the conical intersection of the 1 state with the S0 state provide the mechanism for the ultrafast radiationless deactivation of the excited singlet states of adenine.
TL;DR: Both optical and tunneling spectra show that the level structure depends primarily on the diameter of the rod and not its length, and with increasing diameter, the band gap and the excited state level spacings shift to the red.
Abstract: Photoluminescence excitation spectroscopy and scanning-tunneling spectroscopy are used to study the electronic states in CdSe quantum rods that manifest a transition from a zero-dimensional to a one-dimensional quantum-confined structure. Both optical and tunneling spectra show that the level structure depends primarily on the diameter of the rod and not its length. With increasing diameter, the band gap and the excited state level spacings shift to the red. The level structure was assigned using a multiband effective-mass model, showing a similar dependence on rod dimensions.