FOLLOWING THE CHANGES IN LOCAL GEOMETRY ASSOCIATED WITH A SURFACE-REACTION - THE DEHYDROGENATION OF ADSORBED ETHYLENE

Using scanned-energy-mode photoelectron diffraction we have determined the local adsorption geometry of the reactant and product molecules i n a model heterogeneous reaction on a Ni(111) surface in which ethylen e (C2H4) is dehydrogenated to acetylene (C2H2) Although the C-C axis r emains essentially parallel to the surface, a change in adsorption sit e occurs, indicating that for the case of a concerted reaction mechani sm only two distinct pathways are possible. Precise structural informa tion on a model system of this kind has hitherto been unavailable.