XAS Spectra (XA + spectrum)

Distribution by Scientific Domains

Selected Abstracts

EXAFS and molecular dynamics studies of ionic solutions

Paola D'Angelo
This paper focuses on recent advances in the X-ray absorption spectroscopy (XAS) analysis of ionic solutions. The asymmetry of radial distribution functions associated with the solvent molecules surrounding the ions has to be taken into account to perform a reliable structural analysis. Molecular dynamics (MD) simulations provide reliable 's which can be used as starting models in the XAS data analysis. The combined MD-XAS investigation reduces meaningfully the indetermination of the structural parameters, especially for coordination numbers and Debye,Waller factors. Double-electron excitation channels can be present in the XAS spectra of ionic solutions and they have to be accounted for in the background extraction. The ability of the XAS technique to probe three-body correlation functions in ionic solutions with the aid of MD has been shown. The analysis of the low- k region of the spectra allows the detection of a weak but significant hydrogen structural signal. The XAS technique is especially well suited to determine the detailed shape of the nearest-neighbor peak in the atom,atom pair correlation functions of disordered systems. The information that they contain about the short-range atom,atom pairwise interactions can be very helpful for specifying and properly modifying model potentials used in MD simulations. [source]

Symmetry role on the pre-edge X-ray absorption fine structure at the metal K edge

Ziyu Wu
The preedge features in a system with `even' symmetry, apart from quadrupolar transition contribution, are mainly dipolar in character, associated with the existence of unoccupied states made up of mixed cation-4p with higher-neighboring cation-3d orbitals, and reflect the density of states due to the medium-range order of the system. In `odd' symmetry materials these preedge features are the result of a transition from the 1s to a final density of states of p symmetry due to an unsymmetrical mixing of the ligand wave functions with the central cation 3d orbitals, similar to atetrahedral configuration. These results are validated for Fe as a photoabsorber by comparing XAS spectra of Fe2SiO4 (fayalite) to ab initio full multiple scattering calculations at the Fe K edge, but pertain to all systems containing sixfold-coordinated cations. [source]

Electronic state of carbon in carbonaceous chondrite meteorites

A. V. Okotrub
Abstract Electronic state of carbon occurred in the carbonaceous chondrite meteorites has been probed using X-ray emission (XES), X-ray absorption (XAS), and X-ray photoelectron (XPS) spectroscopy. The XES and XAS spectra of seven meteorites Orgueil, Mighei, Murchison, Murray, Staroe Boriskino, Isna, and Kainsaz were compared to the graphite and detonation nanodiamonds spectra. The carbon in the meteorites was found to be presented by the graphite-like component mainly, which ratio to the diamond-like component decreases with the meteorite age. The strong charging effect was found in the XPS C 1s-spectra of the youngest meteorites Kainsaz and Isna that can be due to encapsulation of nanocarbon granules within the dielectric silicate matrix. The presence of the letter species in the chondrites was supported by Raman spectroscopy. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Stabilization of High-Valent FeIVS6 -Cores by Dithiocarbamate(1,) and 1,2-Dithiolate(2,) Ligands in Octahedral [FeIV(Et2dtc)3,n(mnt)n](n,1), Complexes (n=0, 1, 2, 3): A Spectroscopic and Density Functional Theory Computational Study,

Carsten Milsmann Dr.
Abstract A detailed spectroscopic and quantum chemical analysis is presented to elucidate the electronic structures of the octahedral complexes [Fe(Et2dtc)3,n(mnt)n]n, (1,4, n=3, 2, 1, 0) and their one-electron oxidized analogues [Fe(Et2dtc)3,n(mnt)n](n,1), (1ox,4ox); (mnt)2, represents maleonitriledithiolate(2,) and (Et2dtc)1, is the diethyldithiocarbamato(1,) ligand. By using X-ray crystallography, Mössbauer spectroscopy, and Fe and S K-edge X-ray absorption spectroscopy (XAS) it is convincingly shown that, in contrast to our previous studies on [Fe(cyclam)(mnt)]1+ (cyclam=1,4,8,11-tetraazacyclotetradecane), the oxidation of 1,4 is metal-centered yielding the genuine FeIV complexes 1ox,4ox. For the latter complexes, a spin ground state of S=1 has been established by magnetic susceptibility measurements, which indicates a low-spin d4 configuration. DFT calculations at the B3LYP level support this electronic structure and exclude the presence of a ligand , radical coordinated to an intermediate-spin ferric ion. Mössbauer parameters and XAS spectra have been calculated to calibrate our computational results against the experiment. Finally, a simple ligand-field approach is presented to correlate the structural features obtained from X-ray crystallography (100,K) with the spectroscopic data. [source]