PNNL

Abstract

Reduction of UVI by the beam during X-ray photoelectron spectroscopy (XPS) is a commonly observed phenomenon. This can affect the results for valence state determinations of uranium compounds and also compromise the validity of peak parameters derived from UVI standards. However, there is little quantitative information on the reduction kinetics and species produced. The objective of this contribution is to investigate and quantify the effects of X-ray beam reduction of UVI during XPS analysis. 36 successive U4f XPS spectra were taken over a 26 hour period during the X-ray induced reduction of UVI sorbed to a mica substrate. Reduction was marked by correlated and systematic increases in U5f density, decreases in binding energy, and the emergence of satellites commonly associated with UIV and UV. Factor Analysis identified three dominant and, by definition, linearly independent components. Consequently, we fit the U4f level, including the satellite structure, with three components that represented UVI, UV, and UIV. Component parameters were initially derived from standard compounds, but optimum fits required additional degrees of freedom. Nonetheless, peak parameters were remarkably stable and consistent with UVI, UV, and UIV over the entire reduction sequence. Insights from modeling kinetic data for the time evolution of UVI, UV, and UIV are also discussed.