Study Of Electronic Excitations Research Articles

Combining multilevel Hartree-Fock and multilevel coupled cluster approaches with molecular mechanics: a study of electronic excitations in solutions.

We investigate the coupling of different quantum-embedding approaches with a third molecular-mechanics layer, which can be either polarizable or non-polarizable. In particular, such a coupling is discussed for the multilevel families of methods, in which the system is divided into an active and an inactive orbital space. The computational cost of the resulting three-layer approaches is reduced by treating the long-range interactions at the classical level. The developed methods are tested by the calculation of excitation energies of molecular systems in aqueous solution, for which an atomistic description of the environment is crucial to correctly reproduce the specific solute-solvent interactions, such as hydrogen bonding. In particular, we present the results obtained for three different moieties - acrolein, pyridine and para-nitroaniline - showing that an almost perfect agreement with experimental data can be achieved when the relevant physico-chemical interactions are included in the modeling of the condensed phase.

TD-DFT and DFT/MRCI study of electronic excitations in Violaxanthin and Zeaxanthin

We report vibrationally broadened Franck–Condon (FC) spectra of Violaxanthin (Vx) and Zeaxanthin (Zx) for the lowest-energy 1Ag→1Bu band that arises from the bright HOMO→LUMO single-electron excitation. Geometries were optimized using standard (1Ag) and time-dependent (1Bu) density functional theory (DFT) at the (TD-)CAM-B3LYP/6-31G(d) level, both in the gas phase and in acetone using a polarizable continuum model (PCM). DFT/MRCI multireference calculations were performed at the optimized (TD)-CAM-B3LYP structures to evaluate the energies of doubly excited states that are not accessible to linear response TD-DFT theory. The FC spectra were calculated using the time-independent (TI) scheme. The calculated spectra of Vx and Zx are very similar, with a red shift of about 0.1eV for Zx relative to Vx, which is in agreement with the experimental data. The predicted spectral peaks of Vx and Zx deviate from experiment by less than 0.1eV when performing the calculations in the gas phase. In the presence of acetone (PCM model), there are larger deviations so that a state specific correction scheme needs to be applied, which accounts for non-equilibrium solvent relaxation. The 1Ag→1Bu vertical absorption energies and the corresponding vertical fluorescence energies from TD-CAM-B3LYP and DFT/MRCI agree reasonably well. The DFT/MRCI absorption and fluorescence energies for the doubly excited 2Ag and 2Bu states are found to be rather sensitive to the underlying geometry, in particular to the bond length alternation in the polyene chain. In acetone (PCM), Vx and Zx show little bond alternation, and thus the doubly excited Bu state becomes the lowest excited Bu state. (TD)-CAM-B3LYP appears to be suitable for generating realistic geometries for higher-level calculations in such molecules.

European synchrotron radiation facility upgrade beamline UPBL6 – inelastic scattering

As part of phase 1 of the European synchrotron radiation facility (ESRF) upgrade programme, a new beamline (UPBL6) for the study of electronic excitations using inelastic scattering and emission spectroscopy will be designed and constructed. The new beamline will provide an intense stable X-ray beam at two different spectrometers to be used on a time-shared basis. One of the spectrometers will be dedicated to resonant inelastic X-ray scattering (RIXS); the other to X-ray Raman spectroscopy (XRS). This beamline is currently (June 2010) in the design phase and may be fully operational by early 2013.

Time resolved luminescence of solids excited by femtosecond VUV pulses and synchrotron radiation

AbstractTime‐resolved luminescence spectroscopy has been widely used in studies of electronic excitations and their dynamics in insulators. Present paper discusses the potential of vacuum ultraviolet spectroscopy applied for wide band gap crystals using classical synchrotron radiation as well as more recently developed pulsed femtosecond short‐wavelength light sources. Optical spectroscopy using novel free electron lasers and laser systems based on higher harmonic generation process allow detailed investigation of the interaction between electronic excitations and behaviour of solids under extreme radiation conditions. Significant changes in the quantum yield and emission decay kinetics as well as surface damage of single crystals were observed for CdWO4 and CaWO4 crystals studied in the temperature range of 8‐300 K. However, in the range of excitation densities applied, up to ∼1012 W/cm2, the nature of luminescence centres and their characteristic emission spectra remained similar to those observed at low excitation densities. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Inelastic X-ray scattering: new possibilities for Raman spectroscopy

Inelastic X-ray scattering with meV resolution has recently become available for studies of dynamical properties and elementary excitations in solids. Contrary to Raman spectroscopy at visible wavelengths, which in first order is limited to the Brillouin-zone center, the wave vectors in hard X-ray Raman scattering are very large, and the crystal-momentum transfer to elementary excitations, whose energies may range from a few meV up to several eV, can be tuned continuously across the whole Brillouin zone. This paper reviews new and unique possibilities offered by X-ray Raman spectroscopy for crystalline solids, such as phonon-dispersion measurements (GaN), the determination of phonon self-energies (isotopically mixed diamond), and resonance effects and studies of electronic excitations (copper oxides).

Study of electronic excitation and electron capture processes in the collision between ground-state rubidium atoms and potassium ions by crossed beams

Collisional processes between K ions and Rb (both in their ground state) have been studied using a crossed beam technique and measuring the decay fluorescence. Emissions corresponding to decay of the first excited state of Rb and the first two excited states of neutral K have been detected. For these, absolute total cross-section dependence on collision energy, branching ratios between J-states and true excitation cross-sections (from polarization measurements) have been determined, all in the 0.3–4.1 keV range. The results are interpreted using adiabatic potential energy curves calculated ab initio employing one-electron, non-empirical, relativistic pseudopotentials and including core polarisation.

An experimental study of electronic excitation and electron capture processes in Rb(52S1/2) collision with Cs+ and Li+ ions

The collisional systems Rb–Cs+ and Rb–Li+ (with all species in the ground entrance channel) have been studied using the crossed beams technique combined with decay fluorescence measurements. In the former system, only the emissions from the excited atoms Rb(52P1/2,3/2) and Cs(72P1/2,3/2) have been observed, while for the latter the electron exchange channels Li(22P1/2,3/2) and Li(32P1/2,3/2) were present, all of them giving signal strengths much lower than those shown by the same processes for other alkali atom–alkali ion collisional pairs.The collision energy–excitation cross section dependence for all four channels has been determined, as well as the branching ratio between J-states for those involving Cs ions as projectiles. A qualitative interpretation of the observed inelastic processes is performed using abinitio one-electron calculations.

Optical design and performance of the inelastic scattering beamline at the National Synchrotron Light Source

Phase I of the X21 beamline at the National Synchrotron Light Source was commissioned during 1993. The research program at the X21 beamline is focused on the study of electronic excitations in condensed matter with total energy resolution of 0.1–1.0 eV. The source is a 27-pole hybrid wiggler. A water-cooled horizontal focusing Si(220) monochromator and a spherically bent Si(444) analyzer were installed and commissioned. At 8 keV the energy resolution of the monochromator is about 0.7 eV, and the energy resolution of the analyzer is about 0.1 eV. Results from several selected experiments are also discussed.

A difference technique for the study of electronic excitations

A new method for the study of electronic excitations, Excited State Electron Density Differential (ESEDD), in which an excitation process is described as a difference in charge density between excited and ground states, has been developed and applied to the HOMO to LUMO and HOMO to antibonding 1b2g transitions of ethylene.

Corrigendum

Proc. R. Soc. Lond . A 352, 419-428 (1977) High resolution studies of electron excitation. IV. The n = 3 states of helium By D. W. 0. Heddle, R. G. W. Keesing and A. Parkin Three polarization data points were omitted from the block of figure 2 (p. 420). The complete figure is given below: the three points are those at the lowest energies.

High resolution studies of electron excitation V. The metastable states of helium

High energy resolution observations of the threshold region of the He(2 3 S) electron excitation function have been made by monitoring the He(2 3 S) metastable flux. Structure has been observed in the total cross section very close to threshold which is interpreted as resonant s-wave scattering from the 2 3 S state. Measurements of the relative total metastable production cross section have been extended to cover the whole range of energies from the 2 3 S threshold to the ionization potential and the resonant structures associated with the n = 3 and n = 4 states correlated with those observed in the 3 3 S and 3 1 S optical excitation functions.

High resolution studies of electron excitation III. Polarization near threshold of light from the 4D states of helium

We have measured the polarization of the helium lines at 492.2 nm (4 1 D-2 1 P) and 447.2 nm (4 3 D-2 3 P) over an energy range of about 0.8 eV above threshold using velocity selected electrons. The measurements are free from the effects of instrumental polarization. In both cases we find that the value of the polarization at the lowest energies for which we can detect a signal is quite consistent with the threshold value predicted by conservation of angular momentum. At higher energies we find that the polarization falls rapidly. We suggest that this rapid fall is a consequence of a 1s 4d 2 2 S resonance just below the threshold.