X-ray photoelectron spectroscopy investigation of the initial oxygen adsorption sites on the LaB6(100) surface

Abstract

X-ray photoelectron spectroscopy was used to investigate the initial stages of oxygen adsorption on the (100) surface of a single crystal of lanthanum hexaboride. Numerous previous studies had not resolved the issue of whether oxygen adsorbs at lanthanum sites, boron sites, or both. We find that oxygen adsorption markedly alters the La3d lineshapes, whereas the B1s peak is unaffected. On the clean surface the La3d3/2 peak is split into two components at binding energies of 854.7 and 851.8eV, a splitting that is typical of rare-earth metals and their compounds. The two components are associated with two different final states. In one final state the 3d core hole is poorly screened (854.7eV) and in the other it is well-screened (851.8eV). The relative intensity of the two components is known to be very sensitive to the chemical environment of the rare earth atom and a 10L O2 exposure at room temperature produces a large increase in the relative intensity of the well-screened component. Annealing the surface to 600°C and then to 700°C produces sharp c(2×2) and p(2×1) LEED patterns respectively. The La3d peaks associated with the two LEED patterns are 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 is 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.