5f -electron localization in uranium binary hydrides: Photoelectron spectroscopy

The relation of $5f$-electron localization and valence-band x-ray and ultraviolet photoelectron spectra (XPS, UPS) is analyzed on the example of uranium binary hydrides UH₃ and UH₂. Confronted with results of density functional theory (DFT) and DFT+U calculations, it is recognized that electron-electron correlations play an important role. The spectra can be well accounted for by dynamical mean-field theory (DMFT) calculations providing insight into the final state remaining after the photoexcitation event. It is concluded that details of the spectra reflect the dominant final-state 5f² multiplets with the main line ³H₄ adjacent to the Fermi level and the first excited line ³F₂ responsible for a shoulder at 0.5 eV binding energy. Calculations with a varied strength of hybridization between $5f$ and non-f states allow us to visualize how the individual multiplet lines shift in energy. The one-to-one correspondence between features seen in the DMFT calculations and free-ion multiplets remains noticeable even with the realistic strength of the hybridization included, but the positions of some of the lines can vary substantially. The width of the distribution characterizing fluctuations of the $5f$ filling among the individual integral 5fN states, approximated by a Gaussian distribution with a certain width, is suggested as a quantification of the $5f$ delocalization. In this respect, the hydrides are found to be more delocalized than UGa₂ or δ-Pu.