Theory of X‐Ray Absorption Near Edge Structure

This chapter discusses the theory of the x-ray absorption near edge structure (XANES). These processes are evidently very closed with the extended part, and extended x-ray absorption fine structure (EXAFS) theory can be seen as an approximation of the XANES formulation. To introduce the x-ray absorption phenomena, the chapter presents some typical examples showing the sensitivity of this technique. From the interaction Hamiltonian between the electromagnetic-field and one electron, it then gives the transition rate between two states. The chapter presents the general equation for the absorption process. From the characteristic times governing the process involved, it explains which theory, mono-electronic or multi-electronic is the most appropriate to understand the spectra. The chapter also shows the way to calculate them. It then discusses the mono-electronic approach with some emphasis on the multiple scattering theory (MST). Finally, the chapter gives the main aspects of multiplet theory to handle multi-electronic phenomena.