Theoretical study of cluster size effects on X-ray absorption and resonant X-ray emission spectra in d and f electron systems.

Abstract

Study of resonant X-ray emission spectroscopy (RXES) has recently been a subject of remarkable progress with high brightness synchrotron radiation sources. In RXES, a core electron is excited by the incident X-ray photon to the absorption threshold and this excited state decays by emitting an X-ray photon. Therefore, RXES is a second order optical process, whose intermediate state is the same as the final state of the X-ray absorption spectroscopy (XAS), which is a typical first order optical process. Theoretical analysis of XAS and RXES in d and f electron systems, such as transition metal compounds and rare earth compounds, has so far been made with a cluster model (or an impurity Anderson model) including a single cation (denoted hereafter as single-cation model). For nominally d ° and f0 systems, TiO2 and CeO2 for instance, XAS and RXES of CeO2 are well analysed by the single-cation model (see for instance a short review by Kotani (1997), but for TiO2 (Tezuka et al., 1996) such an analysis for RXES breaks down even though it works well for the analysis of XAS (Okada & Kotani, 1993). The spatial extension of the transition metal 3d wave function is larger than that of the rare earth 4 f wave function. Therefore, it is expected that the RXES of transition metal compounds are more sensitive to the cluster size. Furthermore, it is interesting to compare the cluster size dependence of RXES and XAS. It is the purpose of this paper to perform a model study of cluster size effects on XAS and RXES.