Influence of sulfur on oxygen adsorption onto a polycrystalline Zr surface

Oxidation of a polycrystalline Zr surface is studied by a time-of-flight el ectron-stimulated desorption (TOF-ESD) method and Auger electron spectrosco py (AES). An S-segregated Zr surface is obtained after heat treatment at 11 00 K for 15 min in an ultrahigh-vacuum (UHV) chamber. Several cycles of Ar bombardment at 925 K and heating at 1000 K remove sulfur layers, resulting in a nearly clean Zr surface. Although oxygen molecules in dissociative st ates adsorb onto both types of sample surfaces, the sticking coefficient of the oxygen atoms is considerably decreased on the S-segregated surface. Th ree different sites for the adsorption of oxygen atoms are noted, namely, t he subsurface sites, the hollow sites, and the on-top sites. The S atoms se gregate to the Zr surface to occupy the hollow or the on-top site, resultin g in the observed decrease of the sticking coefficient of the O atoms. (C) 2000 Published by Elsevier Science B.V.