Interaction of water vapor with UO₂(001)

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 6.8 eV on the high binding energy side of the U 4f_5/2 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 approx. 25 eV, and the kinetic energy distribution is peaked at 3.5 eV. The O⁺ desorption threshold is correlated to the ionization of the oxygen 2s level, rather than the U 6p levels. Upon exposure to D₂ ¹⁸O at 300 K, 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 20 L 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.