A SIMS and classical dynamics study of the chemisorption of CO on Ni{7911}

The dependence of secondary ion intensities on azimuthal angle of incidence of the primary ion beam in SIMS experiments on a stepped Ni{7911} surface with adsorbed CO has been examined in detail. The surface was examined after various exposures to CO and at temperatures of 231 and 300 K. The angular anisotropies for the Ni⁺, Ni₂⁺, Ni₃⁺, Ni₂CO⁺, and especially the NiCO⁺ species were found to be quite sensitive to surface structure changes suggested using other surface science techniques. Ion yield ratios as a function of exposure of CO to the surface, however, were found to be nearly insensitive to these structural changes. Of particular significance was the presence of a sharp feature in the NiCO⁺ ion yield at an Ar⁺ ion angle of incidence of φ=110° with the crystal temperature at 231 K and after a CO exposure adequate to populate the step edge sites. At higher exposures or temperatures, this feature was washed out when presumably mostly terrace sites are occupied. Using these ion yield versus azimuthal angle curves, it is also apparent that the saturation coverage structures at 231 and 300 K are different. By introducing 10^-7 Torr of CO into the chamber at 300 K the ion yield curves are identical to those for the saturation coverage at 231 K, indicating similar surface structures. Classical dynamics calculations aimed at modeling the structure corresponding to a CO exposure of 0.6 L at 231 K support the idea that the CO molecules reside in twofold bridge sites along the bottom of the step edge.