THE LOCAL ADSORPTION STRUCTURE OF SO2 ON NI(111) - A NORMAL INCIDENCEX-RAY STANDING WAVE-FIELD 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} s catterer planes both parallel to, and at 70 degrees to, the surface pl ane. 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 h igh symmetry sites. S K-edge NEXAFS confirms an earlier finding that t he molecule lies with its molecular plane parallel to the surface. The detailed sites can only be reconciled with a model in which the SO2 m olecules are centred close to hollow sites (with equal occupation of b oth types of hollow) and the internal conformation of the molecule, es pecially the O-S-O bond angle, is significantly different from that of the gas-phase molecule. Specifically, the O-S-O bond angle is estimat ed 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 unus ual pi-bonding (for which there appears to be no analogue in coordinat ion compounds) and thus partial occupation of the 3b(1) pi LUMO of th e molecule. (C) 1997 Elsevier Science B.V.