Quantifying small changes in uranium oxidation states using XPS of a shallow core level

The U 4f line is commonly used to determine uranium oxidation states with X-ray photoelectron spectroscopy (XPS). In contrast, the XPS of the shallow core-levels of uranium are rarely recorded. Nonetheless, theory has shown that the U 5d (and 5p) multiplet structure is very sensitive to oxidation state. In this contribution we extracted the U(IV) and U(V) 5d XPS peak shapes from near stoichiometric and oxidized UO2 single crystal samples, respectively, where the oxidation state of U was constrained by fitting the 4f line. The empirically extracted 5d spectra were similar to the theoretically determined multiplet structures and were used, along with the relatively simple U(VI) component that was constrained by theory, to determine the oxidation states of UO2+x samples. The results showed a very strong correlation between oxidation states determined by the 5d and 4f line and suggested that the 5d might be more sensitive to minor amounts of oxidation than the 4f. Limitations of the methodology, as well as advantages of using the 5d relative to the 4f line are discussed.