Excited state electronic structure and the theory of x-ray spectra

There has been dramatic progress in recent years both in efficient calculations and in the interpretation of various x-ray spectroscopies. For example, real-space multiple-scattering theory gives a unified treatment of extended x-ray absorption fine structure, near edge structure, x-ray magnetic circular dichroism and other spectroscopies. Here we discuss the theory of x-ray spectra in the framework of excited state electronic structure. These are closely connected since x-ray spectra are directly related to a Green function for the excited photoelectron in the presence of a core hole. However, corrections to the independent electron approximation are needed to account for many-body effects such as inelastic losses, screening, and core-hole effects. We discuss recent approaches for calculating such corrections and show that they can explain some of the remaining discrepancies between theory and experiment.