OXYGEN-ADSORPTION ON A W(100) SURFACE STUDIED BY REFLECTANCE SPECTROSCOPY AND ELECTRON-ENERGY-LOSS FINE-STRUCTURE

Oxygen adsorption on a W(100) surface has been investigated by means o f a multichannel differential reflectance spectroscopy and electron en ergy-loss fine structure (EELFS). The EELFS above the oxygen K-edge wa s measured at the exposure of 0.5 ML oxygen. The nearest inter-atomic distance between adsorbed oxygen and tungsten on the top layer was fou nd to be 0.165 nm. This value supports the model that the oxygen is si tuated on the threefold sites and forms WO2. Real-time differential re flectance spectroscopy spectra of p-polarized light incident near the Brewster angle in the photon-energy ranging from 1.5 to 5.2 eV were me asured during the exposure up to 1.1 ML. The variation of the surface dielectric function for an electric field tangential to the surface re veals that optical transitions around 2.6, 3.9 and 4.7 eV are enhanced below 0.7 ML. The enhanced transitions are related to electronic stat es originating from adsorbed oxygen at the tungsten surface. (C) 1998 Elsevier Science B.V. All rights reserved.