Interaction of water vapor with UO2(001)

Abstract

We have studied the interaction of water vapor with UO2(001) by using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS) and electron stimulated desorption (ESD). The clean UO2(001) exhibits ordered c(2×2) LEED patterns in agreement with previously published data. The X-ray photoelectron spectrum of the U 4f peaks exhibits a clear single satellite at 6.8eV on the high binding energy side of the U 4f5/2 feature, which has been assigned to a shake-up excitation of an electron from the O 2pU bonding band to partially occupied or unoccupied localized metal 5f levels. Electron stimulated desorption (ESD) measurements of clean UO2 indicate the desorption of H+ and O+; the O+ desorption threshold is at ∼25eV, and the kinetic energy distribution is peaked at 3.5eV. The O+ desorption threshold is correlated to the ionization of the oxygen 2s level, rather than the U 6p levels. Upon exposure to D218O at 300K, the XPS and ESD results show no evidence of OD formation after water exposure. However, an ESD 18O+ signal appears; both 16O+ and 18O+ signals reach a saturation value after 20L exposure. Further, LEIS spectra show an 18O peak after water dosage. These results indicate that the water is completely dissociated on UO2(001) and that 18O atoms are incorporated in the surface. The results also suggest a diffusion of oxygen into the bulk.