The local adsorption structure of SO2 on Ni(111): a normal incidence X-ray standing wavefield determination

A normal incidence X-ray standing wavefield study of the structure of molecular SO2 on Ni(111) has been conducted, using photoabsorption at both the O and S atoms and real-space site triangulation using {111} scatterer planes both parallel to, and at 70 degrees to, the surface plane. Both O and S atoms are found to be in the vicinity of atop sites, although the S atoms are displaced significantly further from these high symmetry sites. S K-edge NEXAFS confirms an earlier finding that the molecule lies with its molecular plane parallel to the surface. The detailed sites can only be reconciled with a model in which the SO2 molecules are centred close to hollow sites (with equal occupation of both types of hollow) and the internal conformation of the molecule, especially the O-S-O bond angle, is significantly different from that of the gas-phase molecule. Specifically, the O-S-O bond angle is estimated to be no more than 100 degrees, while the data indicate an S-O bond-length expansion of 5% or more. This change is attributed to the unusual pi-bonding (for which there appears to be no analogue in coordination compounds) and thus partial occupation of the 3b(1) pi* LUMO of the molecule. (C) 1997 Elsevier Science B.V.