THE ADSORPTION SITE AND ORIENTATION OF CH3S ON NI(111)

The angle-resolved X-ray photoelectron spectra for 0.15 monolayers (ML ) of sulfur, and 0.25 ML methyl thiolate formed at 100 K and annealed to 150 and 250 K, on Ni(lll) are analyzed to determine the structures of these species. It is found that sulfur adsorbs on the face-centered cubic hollow site on Ni(111) with a S-Ni bond length of 2.20 +/- 0.02 Angstrom. The thiolate species formed at 150 K has the C-S bond tilte d at similar to 35 degrees to the surface normal with a C-S bond lengt h of 1.85 +/- 0.02. Angstrom and a S-Ni bond length similar to that fo r adsorbed sulfur (2.2 Angstrom). The methyl group is tilted toward th e bridge site and the thiolate appears to be adsorbed on the face-cent ered cubic site although there may also be adsorption in the hexagonal close packed site. The species formed at 250 K adsorbs on a reconstru cted surface where the chemical shift of the S 2p core level indicates that it adsorbs at a four-fold site and the angle-resolved XPS data i ndicate that the C-S bond is oriented normal to the surface. The calcu lated angular variations in intensity are consistent with this interpr etation but cannot distinguish between the various models proposed for the reconstructed surface. (C) 1997 Elsevier Science B.V.