STRUCTURAL ASPECTS OF THE INTERACTION OF METHYL THIOL AND DIMETHYLDISULPHIDE WITH NI(111)

A combination of near-edge and surface extended x-ray absorption fine structure (NEXAFS and SEXAFS), normal incidence x-ray standing wavefie ld absorption (NIXSW) and (laboratory-based) x-ray photoelectron spect roscopy and diffraction (XPS and XPD) has been used to investigate the local phases and associated structures resulting from the interaction of methylthiol (CH3SH) and dimethyldisulphide ((CH3S)(2)-DMS) with Ni (111). The interaction was followed by adsorbing at low temperature an d observing the effects of sequential heating cycles to up to 373 K. A t the lowest temperature DMS appears to be intact on the surface, but at slightly higher temperatures, up to at least 223 K, a second molecu lar species is formed which is also obtained by the thiol adsorption, even at 153 K. This is identified as methylthiolate (CH3S-) and appear s to be bonded (through the S atom) to the hollow sites on the surface with a S-NI bondlength of 2.24 Angstrom; essentially equal occupation of both (fcc and hcp) hollow sites is found. Both XAFS and XPD indica te that this species shows a substantial tilt angle of the S-C symmetr y axis away from the surface normal; the situation is somewhat similar to that reported for the same species on Pt(111) on the basis of C K- edge NEXAFS and vibrational spectroscopy, and comparative S K-edge NEX AFS measurements for Pt(111) are also reported here. The tilted geomet ry found on both Ni and Pt surfaces contrasts with the perpendicular o rientation found on Cu(111). Heating the Ni(111) surface to 373 K conv erts the thiolate species to a third state which we identify as atomic sulphur. Both SEXAFS and NEXAFS show that this final species has the same local geometry as that found in earlier studies of atomic S on Ni (111): in particular, while the S-Ni nearest-neighbour spacing is simi lar to that of the thiolate, the S in this phase appears to penetrate the top Ni layer.