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Full vibrational characterization of ethylene adsorption on Si(001)-(2x1) by a combined theoretical and experimental approach

The vibrational and structural properties of a single-domain Si(001)-(2x1) surface upon ethylene adsorption have been studied by density functional cluster calculations and high-resolution electron energy loss spectroscopy. The detailed analysis of the theoretically and the experimentally determined vibrational frequencies reveals two coexisting adsorbate configurations. The majority ethylene species is di-sigma bonded to the two Si atoms of a single Si-Si dimer. The local symmetry of this adsorption complex for ethylene saturation is reduced to C2 as determined by surface selection rules for the vibrational excitation process. The symmetry reduction includes the rotation of the C-C bond around the surface normal and the twist of the methylene groups around the C-C axis. Experimentally 17 ethylene-derived modes are found and assigned for the majority and the minority species based on a comparison with calculated vibrational frequencies. The minority species which can account up to 14 % of the total ethylene coverage is spectroscopically identified for the first time. It is assigned to ethylene molecules di-sigma bonded to two adjacent Si-Si dimers (in an end-bridge configuration). One part of these minority species desorbs molecularly at 665 K, about 50 K higher than the majority species, whereas the remaining part dissociates to adsorbed acetylene at temperatures around 630 K. For the latter a di-sigma end-bridge like bonding configuration is proposed based on a comparison of the vibrational spectra with data for adsorbed acetylene on Si(100)-(2x1).