  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).
