The local structure of SO2 and SO3 on Ni(111)

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(111). 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 S1s 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 Is and O Is 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 C-3v 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 A closer to the surface than the S atom. This general bonding orientation for SO3 is similar to that found on Cu(111) and Cu(100) both experimentally and theoretically, although the detailed adsorption sites differ. (C) 2005 Elsevier B.V. All rights reserved.