Dipole-forbidden 5d-5f Electronic Structure in Actinide Systems

We report the first measurements of nonresonant inelastic x-ray scattering (NIXS) from the semi-core 5$d$ levels of several actinide compounds. We find that the lowest energy electronic excitations form a rich spectrum of resonances having quite large angular momentum and whose general characteristics are predominantly dependent on the valence electron configuration rather than the nominal valence of the excited species. Comparison with atomic multiplet calculations strongly supports these observations while also illustrating a new complexity to the excited-state spectrum of actinide compounds: good agreement with experiment, subject to realistic considerations about screening effects, are achieved only when one includes the configuration interaction between the 5$f$ and 6$f$ states.