Change in surface polarizability of Mo(100) with oxidation measured by XPS of physisorbed xenon

Abstract

The polarizability of a Mo (100) surface has been characterized as a function of surface oxidation using XPS measurements of the extra-atomic relaxation energy EPER in physisorbed xenon. The clean and oxidized molybdenum surfaces as well as the physisorbed Xe were prepared and characterized in UHV by XPS. To unambiguously obtain the Xe relation shifts, we have measured the change in the Auger parameter, Δα, between gas phase and physisorbed Xe (α = Auger kinetic energy minus core level photoelectron kinetic energy). The Auger parameter allows a separation of initial state potential changes from final state relaxation changes. The xenon EPER decreases from 2.55 eV on the clean surface to 1.95 eV on a surface with 1.3 ML of oxygen atoms incorporated into the surface region to 1.8 eV on a MoO2 surface. The decrease in EPER is attributed to a decreased surface polarizability due to a reduction in the conduction band electron density. In contrast, the local electrostatic potential at the xenon adsorption site increases by 3.05 eV and most of the increase accompanies the incorporation of the first monolayer of oxygen atoms. This increase is attributed to a contraction of the surface dipole layer.