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 pr ogress is now being made in understanding X-ray absorption near-edge struct ure (XANES). This presentation reviews the developments in this field by ma ny groups leading up to the current state of the art. These developments ha ve led to several ab initio codes, such as FEFF, which yield results compar able to experimental results for XAFS, and permit an interpretation of the spectra in terms of geometrical and electronic properties of a material. Th e 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 e dge, where full MS calculations are often necessary. However, a fully quant itative treatment of XANES remains challenging, largely as a result of a nu mber of many-body effects, e.g. the approximate treatment of the core-hole, multiplet effects, the photoelectron self energy and inelastic losses. Fin ally, natural extensions of the theory to other spectroscopies, such as ano malous X-ray scattering, DAFS (diffraction anomalous fine structure) and XM CD (X-ray magnetic circular dichroism) are briefly discussed. These develop ments are illustrated with a number of applications.