Theory of X-ray emission spectra in f and d electron systems

X-ray emission spectroscopy (XES) is undergoing remarkable progress with high brightness synchrotron radiation sources. Here I discuss some theoretical aspects of XES in rare earth compounds and transition metal compounds. It is to be noted that XES is formulated as a coherent second order quantum process, in contrast to the fact that X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) are first-order optical processes, because the X-ray emission follows the preceding X-ray excitation of a core electron. When the incident X-ray photon energy, Omega, is well above the threshold of the core electron excitation, the XES corresponds to the normal XES (NXES), but when Omega is near the threshold, it is called the resonant XES (RXES). The final states of the first-order processes, XPS and XAS, are the same as the intermediate states of the second-order processes, NXES and RXES, respectively. I show some examples of RXES, where the atomic multiplet structure, the charge transfer effect and the crystal field level excitation play important roles. The calculated results are compared with recent experimental data, and it is discussed what information on the electronic states can be obtained from the analysis of XES data. (C) 1998 Elsevier Science B.V. All rights reserved.