Local adsorption geometry of acetylene on Si(100)(2x1)

Using C 1s scanned-energy-mode photoelectron diffraction the local adsorpti on geometry of acetylene on the Si(100)(2x1) surface has been determined an d the results are compared with those of a similar study of ethylene adsorp tion on this surface. Both molecules bond to the surface along the Si-Si di mers with the C-C bonds parallel to the surface such that the C atoms are i n off-atop sites relative to the Si dimer atoms. In both cases the Si-Si bo nd length (2.36+/-0.21 Angstrom, for ethylene and 2.44+/-0.58 Angstrom for acetylene) is compatible only with the dimer remaining intact after adsorpt ion and not with the Si-Si distance of an ideally terminated undimerized Si (100) surface (3.84 Angstrom).