CO adsorption on the stepped Ag(2 2 1) surface

Abstract

The adsorption and geometry of CO on the stepped Ag(2 2 1) surface, Ag(S)[4(1 1 1) × (1 1 1)], has been studied by temperature programmed desorption (TPD) and time-of-flight electron-stimulated desorption ion angular desorption (TOF-ESDIAD) methods. The adsorbed CO molecule exhibits two distinct desorption processes near 80 K (α 1) and near 90 K (α 2). At low coverage, only the α 2 desorption feature is observed while at higher coverage the α 1 feature can be seen as well. Upon electron bombardment, three desorption species can be identified by TOF-ESDIAD, O +, CO +, and CO* (electronically excited neutral CO species). The coverage-dependent CO* angular distribution pattern was studied by annealing the surface. At low coverage, a two-beam pattern aligned along the 〈 1 ¯ 1 0 〉 azimuth is obtained indicating that CO molecules tilt in both directions along the step edges. At higher coverage, an additional feature is observed, producing a three beam pattern, which contains contributions both from terrace and step sites. This suggests that CO preferentially adsorbs at the step sites of Ag(2 2 1) at low coverages, and that both terrace and step sites are occupied at higher coverages. It is also concluded that the CO molecules adsorb perpendicular to the terrace surface.