Direct observation of pure pentavalent uranium in U₂O₅ thin films by high resolution photoemission spectroscopy

Thin films of the elusive intermediate uranium oxide U₂O₅ have been prepared by exposing UO₃ precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO₂ to U₂O₅, which is consistent with the 5f ² (UO₂) and 5f ¹ (U₂O₅) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.