Interaction of water vapor with UO2(001)

We have studied the interaction of water vapor with UO2(001) by using low e nergy 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 x 2) LEED patterns in agreement wit h previously published data. The X-ray photoelectron spectrum of the U 4f p eaks 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 excit ation 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+ deso rption threshold is at similar to 25 eV, and the kinetic energy distributio n is peaked at 3.5 eV. The O+ desorption threshold is correlated to the ion ization of the oxygen 2s level, rather than the U 6p levels. Upon exposure to (D2O)-O-18 at 300 K, the XPS and ESD results show no evidence of OD form ation after water exposure. However, an ESD O-18(+) signal appears; both O- 16(+) and O-18(+) signals reach a saturation value after 20 L exposure. Fur ther, LEIS spectra show an O-18 peak after water dosage. These results indi cate that the water is completely dissociated on UO2(001) and that O-18 ato ms are incorporated in the surface. The results also suggest a diffusion of oxygen into the bulk. (C) 2000 Elsevier cience B.V. All rights reserved.