Characterization offelectrons in light lanthanide and actinide metals by electron-energy-loss and x-ray photoelectron spectroscopy

Core-excitation edges in metallic La, Th, and U have been investigated by electron-energy-loss spectroscopy (EELS) and x-ray photoelectron spectroscopy (XPS). Strong excitation-energy-dependent variations of the EELS spectra are observed and shown to be due to the energy dependence of the scattering cross section. An analysis of the Th $4f$ spectra in an atomic scattering picture indicates $5f$ symmetry for the realized final states. Furthermore, for the La $3d$ and $4d$ as well as for the Th $5d$ core-excitation edges, a multiplet calculation in intermediate coupling was carried out, yielding excellent agreement with the observed fine structure in the EELS spectra. Thus, in analogy to La metal, it is concluded that a final state of the type $5d^{9}5f^1$ is observed in the Th-metal $5d$ spectrum. Qualitative arguments for an atomic picture of the core excitations of U are also presented. The knowledge of the final states realized in EELS, together with the measured threshold energies in EELS and XPS, allows a determination of the symmetry of the final-state screening orbitals in XPS. It is shown that the core holes in La are screened mainly by $d$ conduction electrons, while in Th and U the screening is achieved predominantly by $5f$ electrons. It is demonstrated that the combined EELS-XPS technique constitutes a powerful tool for determining the degree of $5f$ localization in actinides and their compounds.