Interaction of O2 with LaB6(0 0 1) surfaces as studied with MIES and UPS

Clean and O2 adsorbed LaB6(0 0 1) surfaces were investigated by metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS). Two structures appear close to the Fermi level EF and 3 eV below it in the MIE spectrum of the clean LaB6(0 0 1) surface (work function; 2.3 eV), and can be assigned to the La(5d) and dangling bond of B(2p), respectively. The peak near EF originates mainly from auto-detachment process between He*(1s2s) and La(5d). The results suggest that the clean LaB6(0 0 1) surface is terminated with La. Particular emphasis was paid to changes of these spectral features as a function of oxygen exposure. Both structures disappear at 1.5 L. It is concluded that O2 is dissociated at the La-terminated surfaces. Two oxygen-induced peaks appear in the MIES and UPS(HeII) spectra at 5.8 and 7.5 eV binding energy. It is concluded that O2 is dissociated at the La-terminated surfaces. Their origin is discussed on the basis of two mechanisms for the interaction of O2 with LaB6 surfaces.