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

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 whethe r oxygen adsorbs at lanthanum sites, boron sites, or both. We find that oxy gen adsorption markedly alters the La 3d lineshapes, whereas the B Is peak is unaffected. On the clean surface the La 3d(3/2) peak is split into two c omponents at binding energies of 854.7 and 851.8 eV, a splitting that is ty pical of rare-earth metals and their compounds. The two components are asso ciated with two different final states. In one final state the 3d core hole is poorly screened (854.7 eV) and in the other it is well-screened (851.8 eV). The relative intensity of the two components is known to be very sensi tive to the chemical environment of the rare earth atom and a 10 L O-2, exp osure at room temperature produces a large increase in the relative intensi ty of the well-screened component. Annealing the surface to 600 degrees C a nd then to 700 degrees C produces sharp c(2 x 2) and p(2 x 1) LEED patterns respectively. The La 3d peaks associated with the two LEED patterns are si milar to those observed after the initial 300 K 10 L O-2, exposure, indicat ing oxygen bonding to La in both overlayer structures. Thus while the XPS d ata clearly reveal oxygen adsorption at La sites, there is no indication of adsorption at boron sites for low O-2, exposures. More extensive oxidation at higher temperatures shows formation of both boron and lanthanum oxides. (C) 1999 Elsevier Science B.V. All rights reserved.