Acetylene adsorption on the Si(001) surface

Using a first-principles pseudopotential technique, we have investigated the adsorption of C2H2 on the Si(001) surface. We have found that, at low temperatures, the di-sigma-bond configuration is the most stable structure from the energetic point of view. According to our calculations C2H2 adsorbs preferentially on the alternate dimer sites, corresponding to a coverage of 0.5 monolayer. Our calculated surface band structure suggests that the end-bridge configuration, recently pointed out as a more favorable configuration by first-principles calculations, presents a metallic character and thus is Peierls unstable. The di-sigma adsorbed system is characterized by symmetric and slightly elongated Si-Si dimers, and by a symmetric C-C bond with length close to the double carbon bond length of the ethylene molecule. Our total-energy calculations suggest that other metastable configurations. like the 1.2-hydrogen transfer. the p bridge and the tetra-sigma model are also possible. Available high-resolution electron-energy-loss spectroscopy experimental data are reinterpreted to support the existence of the tetra-sigma model.