Search results for "Absorption"
showing 10 items of 2701 documents
X-ray absorption near edge spectroscopy at the Mn K-edge in highly homogeneous GaMnN diluted magnetic semiconductors
2006
We have studied by X-ray absorption spectroscopy the local environment of Mn in highly homogeneous Ga 1-x Mn x N (0.06 <x<0.14) thin epilayers grown by molecular beam epitaxy on [0001] SiC substrates. The measurements were performed in fluorescence mode around the Ga and Mn K-edges. In this report, we focus our attention to the X-ray absorption near edge spectroscopy (XANES) results. The comparison of the XANES spectra corresponding to the Ga and Mn edges indicates that Mn is substitutional to Ga in all samples studied. The XANES spectra measured at the Mn absorption edge shows in the near-edge region a double peak and a shoulder below the absorption edge and the main absorption peak after …
An ab initio study of the low-lying 1 A′ electronic states of indene
1999
Abstract The electronic absorption spectrum of indene in the region of the lowest-lying valence π → π * singlet excited states was studied using multi-configurational second-order perturbation theory through the CASPT2 formalism. Four 1 A′ states were found to be the most important for describing the excitation spectrum up to 6.8 eV. Their transition energies (oscillator strengths in parentheses) were computed to be: 4.46 eV (0.0004), 5.02 eV (0.175), 6.14 eV (0.221), and 6.36 eV (0.434). These states can be labelled and described as the four Platt states of indene: 1 L a , 1 L b , 1 B b , and 1 B a , respectively. This description of the spectrum explains the experimental observations bett…
Rydberg or Valence? The Long-Standing Question in the UV Absorption Spectrum of 1,1′-Bicyclohexylidene
2008
The electronic excited states of the olefin 1,1'-bicylohexylidene (BCH) are investigated using multiconfigurational complete active space self-consistent-field second order perturbation theory in its multi-state version (MS-CASPT2). Our calculations undoubtedly show that the bulk of the intensity of the two unusually intense bands of the UV absorption of BCH measured with maxima at 5.95 eV and 6.82 eV in the vapor phase are due to a single pi pi* valence excitation. Sharp peaks reported in the vicinity of the low-energy feature in the gas phase correspond to the beginning of the pi 3s(R) Rydberg series. By locating the origin of the pi pi* band at 5.63 eV, the intensity and broadening of th…
A theoretical study of the electronic spectrum of cis-stilbene
1999
Abstract The valence and Rydberg electronic excited singlet states of cis -stilbene have been studied using multiconfigurational second-order perturbation theory (CASPT2). The study includes a geometry determination of the ground state. The geometry of the lowest 1 B state has also been optimized, leading to a structure where the two benzyl groups are nearly perpendicular. The calculated vertical spectrum is in agreement with the occurrence of three main bands in the observed one-photon absorption spectrum. Below the relatively intense transition to the 2 1 B (HOMO→LUMO) state at 4.61 eV, two weak transitions have been computed at 4.11–4.45 eV, which correspond to transitions to the 1 1 B a…
Theoretical Study of the Electronic Spectrum of Imidazole
1996
The complete active space (CAS) self-consistent field (SCF) method and multireference second-order perturbation theory (CASPT2) have been used to study the electronic spectrum of imidazole and the imidazolium ion. The calculations comprise a large number of, both singlet and triplet, valence and Rydberg excited states. A newly developed continuum model has been used to compute solvatochromic shifts. In the gas phase the first and second π → π* excited singlet valence states of imidazole are computed at 6.72 and 7.15 eV, and they shift to 6.32 and 6.53 eV upon solvation. The gas-phase values are somewhat too large (≈0.3 eV) due to an erroneous valence−Rydberg mixing in the CASSCF wave functi…
Comparative Mössbauer spectroscopic study of iron impurities in ZnSe
2001
Abstract An anomalous spectral doublet attributed to Fe 2+ impurity pairs is observed in Mossbauer absorption spectra of 57 Fe 0.01 Zn 0.99 Se below room temperature. In Mossbauer emission experiments, ZnSe: 57 Co polycrystal and powder sources exhibit below 45 K the nucleogenic Fe 1+ valence state born as an after effect of the electron-capture decay of 57 Co. Above 8 K, the proportion of the nucleogenic Fe 1+ decreases under the effect of the thermally activated Fe 1+ →Fe 2+ relaxation. The activation energy is estimated to 0.004 eV. While nucleogenic Fe 1+ is absent in ZnSe: 57 Co single crystal spectra, increasing proportions are detected in samples with an enhanced specific surface. Th…
Theoretical characterization of the lowest-energy absorption band of pyrrole
2002
The lowest-energy band of the electronic spectrum of pyrrole has been studied with vibrational resolution by using multiconfigurational second-order perturbation theory (CASPT2) and its multistate extension (MS–CASPT2) in conjunction with large atomic natural orbital-type basis sets including Rydberg functions. The obtained results provide a consistent picture of the recorded spectrum in the energy region 5.5–6.5 eV and confirm that the bulk of the intensity of the band arises from a ππ∗ intravalence transition, in contradiction to recent theoretical claims. Computed band origins for the 3s,3p Rydberg electronic transitions are in agreement with the available experimental data, although new…
Femtometer accuracy EXAFS measurements: Isotopic effect in the first, second and third coordination shells of germanium
2009
The analysis of the EXAFS signals from 70Ge and 76Ge has evidenced the low-temperature effect of isotopic mass difference on the amplitude of relative atomic vibrations. This effect is reflected in the difference of the Debye-Waller factors of the first three coordination shells, and on the difference of nearest-neighbour average interatomic distances, evaluated with femtometer accuracy. The experimental results are in agreement with theoretical expectations.
The lineshape of the electronic spectrum of the green fluorescent protein chromophore, part I: gas phase.
2014
The vibronic spectra of the green fluorescent protein chromophore analogues p-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI) and 3,5-tert-butyl-HBDI (35Bu) are similar in the vacuum, but very different in water or ethanol. To understand this difference, we have computed the vibrationally resolved solution spectra of these chromophores, using the polarizable continuum model (PCM) to account for solvent effects on the (harmonic) potential energy surfaces (PES). In agreement with experiment, we found that the vibrational progression increases with the polarity of the solvent, but we could neither reproduce the broadening, nor the large difference between the absorption spectra of HBDI and…
Crosslinked chitosan/poly(vinyl alcohol)-based polyelectrolytes for proton exchange membranes
2019
[EN] The preparation polyelectrolytes based on crosslinked poly(vinyl alcohol) (PVA) and chitosan (CS) was considered as a feasible alternative to develop highly functionalised, cost-effective and eco-friendly membranes for proton exchange fuel cell technologies. CS/PVA-based membranes were combined with sulfosuccinic acid (SSA) as crosslinking and sulfonating agent, and glycerol (GL) to promote flexibility and favour their manageability. The chemical structure, the thermo-oxidative behaviour, the ethanol uptake, the electric, the proton conductivity, and the performance in direct ethanol fuel cell (DEFC) were assessed. In general, all the CS/PVA-based polyelectrolytes showed a synergetic i…