Angular dependence of the vanadium K-edge x-ray appearance near-edge structure (XANES) for the ${\mathrm{VOPO}}_{4}{\mathrm{\ensuremath{\cdot}}2\mathrm{H}}_{2}\mathrm{O}$ xerogel is thoroughly studied both experimentally and theoretically. The main attention is paid to the pre-edge fine structure (PEFS) of the spectra which was shown earlier to be a useful tool for the atomic short order investigations. Good quantitative agreement between theory and experiment obtained for both dipole and quadrupole contributions to the spectra proves validity of the calculation method developed and enables us to ascertain the nature of all the features in the PEFS's. The $p\ensuremath{-}d$ mixture effect due to distortion of the central coordination octahedron and the quadrupole transitions are proved to be the only mechanisms responsible for the PEFS formation in the case considered. We show that in order to achieve quantitative agreement between experimental and theoretical spectra, it is necessary to include the effect of atomic vibrations, which makes the forbidden transitions to molecular orbitals of the central octahedron (MOCO's) dipole allowed, and to take into account deviation of the crystal layers from the substrate plane, which is not a single crystal but a texture.

Angular dependence of the vanadium K-edge x-ray appearance neur-edge structure (XANES) for the VOPO4. 2H(2)O xerogel is thoroughly studied both experimentally and theoretically. The main attention is paid to the pre-edge fine structure (PEFS) of the spectra which was shown earlier to be a useful tool for the atomic short order investigations. Good quantitative agreement between theory and experiment obtained for both dipole and quadrupole contributions to the spectra proves validity of the calculation method developed and enables us to ascertain the nature of all the features in the PEFS's. The p-d mixture effect due to distortion of the central coordination octahedron and the quadrupole transitions are proved to be the only mechanisms responsible for the PEFS formation in the case considered. We show that in order to achieve quantitative agreement between experimental and theoretical spectra, it is necessary to include the effect of atomic vibrations, which makes the forbidden transitions to molecular orbitals of the central octahedron (MOCO's) dipole allowed, and to take into account deviation of the crystal layers from the substrate plane, which is not a single crystal but a texture.

A method based on the local density approximation and on the multiple scattering wave calculation taking into account screened X-ray core hole potential for the final state has been used to analyze the experimental XANES spectra of the VOPO4,2H(2)P xerogel. The effect of cluster size on the computations has been analyzed concluding that the pre-edge depends only on the first shell formed by the six nearest oxygen atoms. In VOPO4,2H(2)O structure, the absorbing VO6 octahedron is surrounded by PO4 tetrahedra. Thus VO6 octahedra are not linked with each other excluding solid state effect in the pre-edge region. In contrast, edge region is sensitive to the cluster size. Computations show that at least a cluster of V5P4O26 must be taken into account to reproduce, in rather good agreement, the edge part of the absorption spectrum.

In this work, we want to clear up the origin of the XAS structures for rutile compounds. To determine the connections between the atomic arrangement and XAS structures, we have calculated the Ti K edge in TiO2 and compared results when few atoms are removed from the cluster. Multiple Scattering approach from FEFF6 code is used.

A method based on the local density approximation and on the multiple scattering wave calculation taking into account screened X-ray hole potential for the final state has been used to analyze the experimental XANES spectra of the TiO2 rutile. The effect of cluster size on the computations has been analyzed concluding that taking into account atoms forming all neighboring octahedra lead to a reasonable agreement between theoretical and experimental spectra. In the crystal structure containing only slightly distorted regular octahedra in ground state, only two peaks corresponding to 1s --> t(2g) and e(g) transitions are obtained. In contrast, due to the effect of the core hole potential calculated self consistently, the locally projected 3d density (DOS) is shifted down by about 3 eV giving rise to quadrupole transition. The 3d DOS arising from neighboring octahedra is not shifted by the core hole potential and contains non-centrosymmetric wave functions arising from solid state effect giving rise to dipole transitions. Temperature vibrations of atoms decrease the symmetry of the system thus making dipolar transitions to t(2g) and e(g) of absorbing octahedron allowed, this increases the intensities of the two first peaks A(1) and A(2) in the pre-edge region.