Theoretical approaches to x-ray absorption fine structure

Dramatic advances in the understanding of x-ray absorption fine structure ( XAFS) have been made over the past few decades, which have led ultimately t o a highly quantitative theory. This review covers these developments from a unified multiple-scattering viewpoint;The authors focus on extended x-ray absorption fine structure (EXAFS) well above an x-ray edge,and, to a lesse r extent, on x-ray absorption near-edge structure (XANES) closer to an edge . The discussion includes both formal considerations, derived from a many-e lectron formulation, and practical computational methods based on independe nt-electron models, with many-body effects lumped into various inelastic lo sses and energy shifts. The main conceptual issues in XAFS theory are ident ified and their relative importance is assessed; these include the converge nce of the multiple-scattering expansion, curved-wave effects, the scatteri ng potential, inelastic losses, self-energy shifts, and vibrations and stru ctural disorder. The advantages and limitations of current computational ap proaches are addressed. with particular regard to quantitative experimental comparisons.