- a test case for actinide materials*

A successful soft x-ray resonant inelastic x-ray spectroscopy (RIXS) experiment of UO₂ at the U N₄ absorption edge (778 eV) will be presented [1]. The RIXS spectra exhibit well resolved excitations due to the crystal-field splitting of the U⁴⁺ 5f ² J=4 (³H₄) Hund's rule ground state, and they show the first excited multiplet (³F₂) at 550 meV. In RIXS the ³H₄→³F₂ transition has a strong cross-section due to the beyond dipole features of the RIXS process which is in strong contrast to the dipole cross-section of inelastic neutron scattering. Hence, the present experiment settles the discussion about the U⁴⁺ multiplet splitting in UO₂. The data are simulated with a full multiplet RIXS calculation, taking into account the RIXS cross-section.

[1] G. H. Lander et al., tbp