The structure of CO on Ni(111)

Abstract

Electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and temperature programmed thermal desorption (TPD) have been used to study the adsorption of CO on Ni(111) in the temperature range 80−300 K. For low coverages, the CO layer is disordered; a c(4×2) pattern appears at coverages ϑ∼0.5, the maximum coverage at 300 K. At temperatures 220–240 K, a well-ordered (√7/2×√7/2)R 19° LEED pattern forms at saturation (ϑ∼0.57). At 80 K, the CO saturation layer is characterized by a ’’complex’’ LEED pattern. Only one binding state is seen in TPD for ϑ≲0.40 (peak temperatures 450–430 K); species having lower desorption temperatures populate at higher coverage. At 300 K adsorption the only ESD ion observed is O+, with desorption centered about the direction perpendicular to the surface. The O+ ion yield shows a maximum at intermediate coverages. CO+ ions are also observed at adsorption temperature < 300 K at higher coverages. The ESDIAD patterns for saturation coverage in the range 80–260 K indicate off-normal ion emission in addition to the normal component. The data suggest that for ϑ≲ 0.5, CO is adsorbed in multiply coordinated sites with the molecular axis perpendicular to the surface. At temperatures < 300 K, a fraction of CO can adsorb in singly coordinated sites as well. Upon heating the CO layer formed at 80 K (ϑ?0.57) the CO+ ion yield maximizes at ∼240 K when the (√7/2×√7/2)R 19° LEED pattern is well defined. The CO+ yield therefore reflects the ordering behavior in the adsorbate layer at temperatures below the onset of desorption and indicates a shift of a fraction of CO molecules from multiply coordinated to singly coordinated sites. Finally, a model is proposed involving resonance charge exchange between adsorbed CO molecules to account for the observed variations of the O+ yield with surface coverage.