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-σ-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-σ 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-σ model are also possible. Available high-resolution electron-energy-loss spectroscopy experimental data are reinterpreted to support the existence of the tetra-σ model.