Resonant inelastic x-ray spectroscopy on UO$_{2}$ as a test case for actinide materials.

Resonant inelastic x-ray spectroscopy at the uranium $N_{4}$ absorption edge at 778 eV has been used to reveal the excitations in UO$_{2}$ up to 1 eV. The earlier (1989) studies by neutron inelastic scattering of the crystal-field states within the $^{3}H_{4}$ multiplet are confirmed. In addition, the first excited state of the $^{3}F_{2}$ multiplet at $\sim$520 meV has been established, and there is a weak signal corresponding to the next excited state at $\sim$920 meV. This represents a successful application of soft x-ray spectroscopy to an actinide sample, and resolves an open question in UO$_{2}$ that has been discussed for 50 years. The technique is described and important caveats are drawn about possible future applications.