X-Ray and ultraviolet photoelectron spectroscopy studies of Uranium(IV),(V) and(VI) exposed to HO-plasma under UHV conditions.

Thin films of UO2, U2O5, and UO3 were prepared in situ and exposed to reactive gas plasmas of O2, H2 and H2O vapour produced with an ECR plasma source (electron cyclotron resonance) under UHV conditions. The plasma constituents were analysed using a residual gas analyser mass spectrometer. For comparison, the thin films were also exposed to the plasma precursor gases under comparable conditions. Surface analysis was conducted using X-Ray and ultraviolet photoelectron spectroscopy before and after exposure, by measuring the U 4f, O 1s core levels and the valence band region. The evolution of the peaks was monitored as a function of temperature and time of exposure. After interacting with water plasma at 400 °C, the surface of UO2 was oxidized to a higher oxidation state compared to when starting with U2O5 while the UO3 film displayed weak surface reduction. When exposed to water plasma at ambient temperature, the outermost surface layer is composed of hexavalent uranium in all three cases.