Local adsorption geometry of acetylene on $\mathrm{Si}\left(100\right)(2\mathrm{}\times 1)$

Using C $1s$ scanned-energy-mode photoelectron diffraction the local adsorption geometry of acetylene on the $\mathrm{Si}\left(100\right)(2\mathrm{}\times 1)$ surface has been determined and the results are compared with those of a similar study of ethylene adsorption on this surface. Both molecules bond to the surface along the Si-Si dimers with the C-C bonds parallel to the surface such that the C atoms are in off-atop sites relative to the Si dimer atoms. In both cases the Si-Si bond length $(2.36\mathrm{}\pm 0.21\AA$ for ethylene and $2.44\pm 0.58\AA$ for acetylene) is compatible only with the dimer remaining intact after adsorption and not with the Si-Si distance of an ideally terminated undimerized Si(100) surface (3.84 Å).