Localization of screening orbitals, local-environment effects, and intermediate valence in core-level spectroscopy: Ce vs Tm

Abstract

We have studied the x-ray absorption spectra (XAS) at the $L_3$ and $L_1$ absorption edges of metallic $\gamma$-Ce, Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$, Ce${\mathrm{Pd}}_3$ and compared with those of Tm metal and intermediate valence TmSe. We analyze the x-ray absorption near edge structure (XANES) taken at high signal-to-noise ratio and high resolution using synchrotron radiation at the facility PULS in Frascati with respect to (1) energy shifts of core-level transitions due to $4f$ electron screening, (2) determination of the local character of the $5d$ final states and mixing of the $5d$ with $6s$,$p$ orbitals near the Fermi level, and (3) use of the XANES as a microscopic tool to determine the intermediate valence in mixed-valence rare-earth-metal compounds. It is concluded that while for the heavy rare earths, Z > Sm, the XANES gives direct information on the intermediate valence, in Ce the occupancy of the $4f^0{\mathrm{}(5d,6sp)\mathrm{}}^4$ configuration is underestimated by about a factor 5 to 6 in favor of the $4f^1{\mathrm{}(5d,6sp)\mathrm{}}^3$ configuration.