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A L Ankudinov

Publications and source records attributed to A L Ankudinov.

7 recordsLinked to original sources
PubMed2003

Development of XAFS theory.

A major goal of theoretical simulations of X-ray absorption fine structure (XAFS) is to provide calculations for the interpretation and analysis of experimental data in terms of geometrical and electronic information. The extended region or EXAFS (50-2000 eV above an absorption edge) contains geometric information about the pair distribution function, i.e. distances to the nearest neighbors and their orientation. The theory of EXAFS is now well understood and has been recently reviewed [Rehr & Albers (2000). Rev. Mod. Phys. 72, 621-654]. The near-edge region (0-50 eV above the edge) or X-ray absorption near-edge structure (XANES) probes the states just above the Fermi level, and contains important electronic information, e.g. the electronic density of states (DOS). This data can be used to obtain the number of electrons or holes in the electronic configuration and spin and orbital moments on a particular atom via sum rules. XANES calculations with our ab initio code FEFF8 [Ankudinov et al. (1998). Phys. Rev. B, 58, 7565-7576] usually give semi-quantitative agreement with experiment, and permits the interpretation of XANES in terms of DOS. However, fully quantitative calculations remain a challenge. Several effects still need to be considered to treat the XANES region. These include non-spherical parts of the scattering potential and many-body effects such as multi-electron excitations, core-hole effects and local field effects (screening of the X-ray field).

Absorptiometry, Photon↗
PubMed2002

New developments in the theory and interpretation of X-ray spectra based on fast parallel calculations.

There has been dramatic progress over the past decade both in theory and in ab initio calculations of X-ray absorption fine structure. Significant progress has also been made in understanding X-ray absorption near-edge structure (XANES). This contribution briefly reviews the developments in this field leading up to the current state. One of the key advances has been the development of several ab initio codes such as FEFF, which permit an interpretation of the spectra in terms of geometrical and electronic properties of a material. Despite this progress, XANES calculations have remained challenging both to compute and to interpret. However, recent advances based on parallel Lanczos multiple-scattering algorithms have led to speed increases of typically two orders of magnitude, making fast calculations practicable. Improvements in the interpretation of near-edge structure have also been made. It is suggested that these developments can be advantageous in structural biology, e.g. in post-genomics studies of metalloproteins.

Absorptiometry, Photon↗
PubMed2001

Pt L-edge XANES as a probe of Pt clusters.

The sensitivity of Pt L-edge XANES to local geometric and electronic structure in various Pt(n) clusters is investigated using the ab initio self-consistent FEFF8 code. Calculations based on FEFF8 are found to be in good agreement with experiment. For pure Pt clusters the XANES can distinguish between 2- and 3-dimensional clusters. Self-consistency is important in determining the variation of XANES with cluster size. The effect of a support is also studied. In Pt-Cl(x) clusters the presence of a Cl-Pt bond leads to a "hybridization peak," i.e., a peak in the Cl d-density of states (d-DOS) mixed with Pt d-states. For Pt-H clusters hydrogen addition is well correlated with the growth of a broad shoulder on the white line. This change is attributed largely to AXAFS, i.e., to a corresponding change in the atomic background absorption.

Journal Article↗
PubMed2001

Progress and challenges in the theory and interpretation of X-ray spectra.

There has been dramatic progress over the past decade both in theory and in ab initio calculations of X-ray absorption fine structure (XAFS). Rapid progress is now being made in understanding X-ray absorption near-edge structure (XANES). This presentation reviews the developments in this field by many groups leading up to the current state of the art. These developments have led to several ab initio codes, such as FEFF, which yield results comparable to experimental results for XAFS, and permit an interpretation of the spectra in terms of geometrical and electronic properties of a material. The review begins with a summary of the key theoretical developments that are essential for achieving a quantitative agreement with experiment for XAFS. The same high-order multiple-scattering (MS) theory of XAFS can also give an approximate treatment of XANES, but this approach can fail close to an edge, where full MS calculations are often necessary. However, a fully quantitative treatment of XANES remains challenging, largely as a result of a number of many-body effects, e.g. the approximate treatment of the core-hole, multiplet effects, the photoelectron self energy and inelastic losses. Finally, natural extensions of the theory to other spectroscopies, such as anomalous X-ray scattering, DAFS (diffraction anomalous fine structure) and XMCD (X-ray magnetic circular dichroism) are briefly discussed. These developments are illustrated with a number of applications.

Journal Article↗
PubMed2001

Hole counts from X-ray absorption spectra.

The interpretation of X-ray absorption spectra in terms of electronic structure has long been of interest. Hole counts derived from such spectra are often interpreted in terms of free-atom occupation numbers or Mülliken counts. It is shown here, however, that renormalized-atom (RA) and cellular counts are better choices to characterize the configuration of occupied electron states in molecules and condensed matter. A projection-operator approach is introduced to subtract delocalized states and to determine quantitatively such hole counts from X-ray absorption spectra. The described approach is based on multiple-scattering theory (MST) and on atomic calculations of a smooth transformation relating the X-ray absorption spectra to local projected densities of states (LDOS). Theoretical tests for the s and d electrons in transition metals show that the approach works well.

Journal Article↗
PubMed2001

Effect of hydrogen adsorption on the x-ray absorption spectra of small Pt clusters.

Hydrogen adsorption on Pt(6)H(n) clusters leads to striking changes in the Pt L(2,3) x-ray absorption spectra. These effects are interpreted using a self-consistent real space Green's function approach. Calculations show that they are due largely to changes in the atomic background contribution to x-ray absorption (i.e., atomic x-ray absorption fine structure) and to reduced Pt-Pt scattering at the edge, while Pt-H multiple scattering is relatively weak. The origin of both effects is traced to the change in the local Pt potential due to Pt-H bonding.

Journal Article↗