The local structure of SO2 and SO3 on Ni(1Â 1Â 1)

The normal incidence X-ray standing wave (NIXSW) technique, supported by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS), has been used to determine the local adsorption geometry of SO2 and SO3 on Ni(1 1 1). Chemical-state specific NIXSW data for coadsorbed SO3 and S, formed by the disproportionation of adsorbed SO2 after heating from 140 K to 270 K, were obtained using S 1s photoemission detection. For adsorbed SO2 at 140 K the new results confirm those of an earlier study [Jackson et al., Surf. Sci. 389 (1997) 223] that the molecule is located above hollow sites with its molecular plane parallel to the surface and the S and O atoms in off-atop sites; corrections to account for the non-dipole effects in the interpretation of the NIXSW monitored by S 1s and O 1s photoemission, not included in the earlier work, remove the need for any significant adsorption-induced distortion of the SO2 in this structure. SO3, not previously investigated, is found to occupy an off-bridge site with the C3v axis slightly tilted relative to the surface normal and with one O atom in an off-atop site and the other two O atoms roughly between bridge and hollow sites. The O atoms are approximately 0.87 Å closer to the surface than the S atom. This general bonding orientation for SO3 is similar to that found on Cu(1 1 1) and Cu(1 0 0) both experimentally and theoretically, although the detailed adsorption sites differ.