THE LOCAL GEOMETRY OF REACTANT AND PRODUCT IN A SURFACE-REACTION - THE DEHYDROGENATION OF ADSORBED ETHYLENE ON NI(III)

Using Cls scanned-energy mode photoelectron diffraction based both on approximate ''direct'' methods and full multiple scattering modelling of spectra recorded in different emission angles covering a total spectral data range of 1200 eV, the local geometry of adsorbed ethylene and adsorbed acetylene on Ni(lll) have been determined in a detailed quantitative fashion. Ethylene adsorbed at low temperature (120 K) lies with its C-C axis parallel to the surface and in an aligned bridge site such that the C atoms lie approximately atop Ni atoms. Heating this surface leads to dehydrogenation of the adsorbed ethylene to adsorbed acetylene, and white the C-C axis remains parallel to the surface, the C-C bondlength and C-Ni layer spacing are reduced, and the acetylene occupies a cross-bridge site with the C atoms directly above inequivalent hollow sites on the surface. Both adsorbed species show C-C bondlengths larger than those of the associated gas-phase molecules, indicating a significant reduction of C-C bond order, in agreement with vibrational spectroscopic data. Possible geometrical reaction paths based on a concerted mechanism connecting the reactant and product are discussed.