Transuranium compounds probed by nonresonant inelastic x-ray scattering

Abstract

While x-ray absorption spectroscopy is mainly governed by electric-dipole transitions, the technique of nonresonant inelastic x-ray scattering (NIXS) offers the possibility to explore higher-order multipole transitions. These transitions obey different selection rules that can reach final states of higher angular momenta, opening complementary spectroscopic perspectives. Here, we investigate the suitability of NIXS to study transuranium compounds. We show that the $K$ edge of the Be encapsulation can be practically fully excluded by using the imaging capabilities of the technique arising from the position of the signals on the multidetector. Experimental results for the multipole transitions at the actinide ${O}_{4,5}$ edges (90--120 eV) in ${\mathrm{UO}}_{2}$, ${\mathrm{NpO}}_{2}$, ${\mathrm{PuO}}_{2}$, and ${\mathrm{Pu}}_{2}{\mathrm{O}}_{3}$ are compared with multielectronic calculations. The spectral features are shown to be very sensitive to the ratio of the triakontadipole and octupole transitions, which could potentially be used to assess the radial expansion of the $5f$ wave function, which is expected to occur in covalent mixing with the O $2p$ states.