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

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 (α1) and near 90 K (α2). At low coverage, only the α2 desorption feature is observed while at higher coverage the α1 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.