Search results for "Spectroscopy"
showing 10 items of 10293 documents
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…
Low-Frequency Relaxation Phenomena in α-LiIO3: The Nature and Role of Defects
2002
The lithium iodate ionic conduction in the polar c-axis direction is studied between 35 and 470 K for crystals grown in various conditions. So far, to separate processes induced by the nature of electrodes, the impedance spectroscopy technique had been used at room temperature with both metallic and ideally polarizable insulating electrodes, so that a relaxation of space charges was clearly identified. Here, the temperature dependence of the hopping ionic conductivity exhibits quite different activation energies well related to the growth conditions. Following low-temperature Raman and thermodynamic experiments, a new approach based on a vacancy diffusion mechanism is proposed. Experimental…
Electron trapping centres and cross sections in LiNbO3studied by57Co Mössbauer emission spectroscopy
1999
Fast electron trapping processes and aliovalent charge states following the 57Co(EC)57Fe decay are studied in undoped, 5.4 mol% Mg-doped and 0.1 mol% Fe-doped LiNbO3 in various thermochemical reduction (TCR) states. Static 57Co Mossbauer emission spectra of congruent Mg:LiNbO3 recorded at T = 4.2 K in external magnetic field of 4.6 T are presented. Trapping cross section ratios are derived for Fe3+Li, Nb5+Li and Mg2+Li. A method to determine trap concentrations for TCR states of LiNbO3 is outlined. The electron-capture distance of the traps is found to be 2.7±1.4 nm. As this is much smaller than the 6 keV Auger-electron penetration depth, it is concluded that the distribution of the alioval…
Chemical effect on the XPS spectra of the valence band and on O KLL and Pd MNN Auger spectra in pumice-supported catalysts
1992
X-ray photoelectron spectra of the valence band and X-ray-excited O KLL Auger transition of pumice-supported Pd and Pt catalysts have been obtained and compared to the corresponding spectra of pumice (a naturally occurring amorphous aluminosilicate). The changes observed indicate interaction between metal and support. The valence region of the catalysts consists of three large peaks: one due to the d band of the metals and the other to the O 2p non-bonding orbitals and bonding-type orbitals formed by a mixing of the atomic orbitals (O 2p, Si 3s and Si 3p) of oxygen and silicon from the support. The intensity decrease of the component due to the bonding orbitals, observed in the catalyst spe…
Mixed-Valence Molecular Unit for Quantum Cellular Automata: Beyond the Born-Oppenheimer Paradigm through the Symmetry-Assisted Vibronic Approach.
2016
In this article, we focus on the electron-vibrational problem of the tetrameric mixed-valence (MV) complexes proposed for implementation as four-dot molecular quantum cellular automata (mQCA).1 Although the adiabatic approximation explored in ref 2 is an appropriate tool for the qualitative analysis of the basic characteristics of mQCA, like vibronic trapping of the electrons encoding binary information and cell-cell response, it loses its accuracy providing moderate vibronic coupling and fails in the description of the discrete pattern of the vibronic levels. Therefore, a precise solution of the quantum-mechanical vibronic problem is of primary importance for the evaluation of the shapes o…
Valence electronic structure of C60: Theoretical analysis of photoemission data
1993
Abstract We present a theoretical investigation of the valence band photoemission spectra of C 60 using the nonempirical valence effective Hamiltonian (VEH) method. The VEH-DOVS curves calculated for the C 60 molecule are found to be in excellent agreement with synchrotron-radiation photoemission spectra reported for C 60 films. A detailed interpretation of all the photoemission bands is performed in the light of the VEH results.
Syntheses, structures and physical characterization of two new three-dimensional mixed-valence hexadecavanadate derivatives
2007
Abstract Two new hexadecavanadate derivatives, (bpy)[Zn(4,4′-bpy)2]2[H4ClV16O38]·6H2O 1 and (bpy)[Co(4,4′-bpy)2]2[H4ClV16O38]·6H2O 2 (bpy = 4,4′-bipyridine), were synthesized under the hydrothermal conditions and structurally characterized by IR, XPS and EPR spectroscopy, redox titration and single-crystal X-ray diffraction. The two compounds are isostructural and crystallize in the tetragonal non-centrosymmetric space group P-4n2 (No. 118) with a = 17.124(2), b = 17.124(2), c = 14.724(3) A, V = 4317.5(12) A3 and Z = 1 for the compound 1, and a = 17.038(2), b = 17.038(2), c = 14.754(3) A, V = 4282.9(12) A3 and Z = 1 for the compound 2. Compounds 1 and 2 were constructed from 4-connected {H4…
Characterization of the non-stoichiometry in lanthanum oxyfluoride by FT-IR absorption, Raman scattering, X-ray powder diffraction and thermal analys…
1997
Abstract The FT-IR absorption, FT-Raman scattering, X-ray powder diffraction (XPD), and thermogravimetry were used to explore the non-stoichiometry in LaOF. The TGA-DTA analyses between 30 and 1500°C showed that the LaO1 − xF1 + 2x phases yielded the stoichiometric LaOF as an intermediate product. The temperature of formation of the LaOF and La2O3 increased with increasing excess of fluoride. The room temperature XPD data in 6.5 ⩽ 2θ ⩽ 121° range were analyzed by the Rietveld profile refinement method and subsequently by the bond valence calculations. All LaO1 − xF1 + 2x phases possess the tetragonal PbFCl-type structure (space group: P4/nmm; Z = 2) while the stoichiometric LaOF has the hex…
Diferrocenyl oligothiophene wires: Raman and quantum chemical study of valence-trapped cations
2011
A combination of Raman spectroscopy and density functional theory calculations is used to describe the structural and spectroscopic properties of the different isomeric cations of diferrocenyl quaterthiophenes. Isomerisation of the thienyl β-positions provides site selective oxidation, which gives rise to species that can interconvert by moving the charge over the bridge. The spectroscopic study allows us to describe a sequence of stationary trapped cationic, either ferrocenyl or thienyl, states which constitutes an energy cascade of accessible sites through which the charge transfer can proceed.