He+ ion-scattering spectroscopy studies of UO2(hkl) surfaces

Structural aspects of the (110), (100) and (111) planar surfaces of UO2 have been studied by 500 eV He+ ISS. Measurements of the He+/U signal as a function of polar, β, and azimuthal,phi, angles of incidence indicate for the (110) that U and O are coplanar: Only one critical blocking angle at βsub(c) = 870 was observed. In a vivid example of sensitivity to surface processes, as the temperature of (110) was raised to where faceting begins, the He+/O signal became very strong and He+U disappeared. For the (100), two He+U βsub(c) were observed and interpreted as resulting from oxygen in two bridge-bonding orientations. Previous (111) results were confirmed and, when extended to include other phi, were in agreement with the center-bonded oxygen model. Difficulties in establishing a defined He+/U scattering mechanism are discussed. (orig.)