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

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