Theory of x-ray absorption and resonant x-ray emission spectra

Resonant X-ray emission spectrum (RXES) has recently been a subject of remarkable progress. Theoretically, RXES is formulated as a coherent second order optical process, where the X-ray emission follows the X-ray absorption (XAS) of a core electron. In this paper I discuss from theoretical viewpoint XAS and RXES in f and d electron systems, which are calculated with an impurity Anderson model. Effects of intra-atomic multiplet coupling and interatomic hybridization are taken into account. RXES is calculated for some rare earth compounds and transition metal compounds as a function of the emitted photon energy omega, and it is shown that the RXES gives us information on the electronic excitations (multiplet states and charge transfer states) in the ground state configuration. When we regard RXES as a function of the incident photon energy Omega with fixed omega, the RXES gives us information on the core electron excited states (with more information than XAS). These results are also compared with available experimental data.