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 E//R**P**E 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, the change in the Auger parameter, DELTA alpha , between gas phase and physisorbed Xe ( alpha equals Auger kinetic energy minus core level photoelectron kinetic energy has been measured. The Auger parameter allows a separation of initial state potential changes from final state relaxation changes. The xenon E//R**P**E 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 MoO//2 surface. The decrease in E//R**P**E is asttributed 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.
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