Effect of translational energy on the molecular chemisorption of CO on Ni(111): Implications for the dynamics of the chemisorption process

Abstract

The effect of translational energy on the molecular chemisorption of CO on a Ni(111) surface is used as a probe of the dynamics of the adsorption process. Initial adsorption probabilities, apparent saturation coverages, spatially resolved Auger coverage profiles, and high resolution electron energy loss spectra of CO deposited on the Ni surface from a supersonic molecular beam are measured as a function of translational energy. It is found that the initial adsorption process for CO molecules incident with energies less than 4 kcal/mol differs from that for molecules incident with higher energies. Molecules with kinetic energies below 4 kcal/mol adsorb with an initial adsorption probability of 0.85±0.04 and a high apparent saturation coverage. Molecules with translational energies between 7 to 30 kcal/mol have an initial adsorption probability of 0.46±0.03, and an apparent saturation coverage approximately half that of the low energy molecules. Since the CO packing density and the final chemisorption states are shown to be independent of incident energy, the two apparent saturation coverages are the result of a difference in the surface area over which the CO molecules are spread. This is verified by spatially resolved Auger coverage profile measurements. Molecules at low energies are initially adsorbed with higher mobility than those incident with larger translational energies. High resolution electron energy loss spectra and thermal desorption spectra show no translational energy-induced dissociation. The frequency shift of the bridge-bonded CO stretching mode measured at the periphery of the molecular beam image shows that the energy-induced difference in the CO mobility during the chemisorption process is qualitatively similar on both the clean and partially CO-covered surface. These results are interpreted as evidence for two adsorption pathways into the molecular chemisorption state. Molecules incident on the surface with low energies are identified as mobile precursor molecules to the molecularly chemisorbed molecules. The precursor molecules have access to the molecular chemisorption state via a low energy pathway. As the incident translational energy is increased beyond the effective 4–7 kcal/mol energy barrier, a new adsorption pathway directly into the less mobile chemisorption state becomes accessible. The natures of the precursor molecule, the effective energy barrier and the low energy pathway are discussed.