X-ray photoelectron spectroscopy was used to investigate the initial stages of oxygen adsorption on the (100) surface of a single crystal (table 1). Numerous previous studies had not resolved the issue of whether oxygen adsorbs at lanthanum sites, boron sites, or both. It was found that oxygen adsorption markedly altered the La 3d lineshapes, whereas the B 1s peak was unaffected. On the clean surface the La 3d3/2 peak was split into two components at binding energies of 854.7 and 851.8eV, a splitting that was typical of rare-earth metals and their compounds. The two components were associated with two different final states. In one final state the 3d core hole was poorly screened (854.7eV) and in the other it was well-screened (851.8eV). The relative intensity of the two components was known to be very sensitive to the chemical environment of increase in the relative intensity of the well-screened component. Annealing the surface to 600C and then to 700C produced sharp c(2x2) and p(2x1) LEED patterns respectively. The La 3d peaks associated with the two LEED patterns were similar to those observed after the initial 300K 10L O2 exposure, indicating oxygen bonding to La in both overlayer structures. Thus while the XPS data clearly reveal oxygen adsorption at La sites, there was no indication of adsorption at boron sites for low O2 exposures. More extensive oxidation at higher temperatures shows formation of both boron and lanthanum oxides.

X-Ray Photoelectron Spectroscopy Investigation of the Initial Oxygen Adsorption Sites on the LaB6(100) Surface. Perkins, C.L., Trenary, M., Tanaka, T., Otani, S.: Surface Science, 1999, 423[1], L222-8