X-Ray and ultraviolet photoelectron spectroscopy studies of Uranium(iv),(v) and(vi) exposed to H2O-plasma under UHV conditions

Abstract

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.