Adsorption geometries and vibrational modes of ${\mathrm{C}}_2{\mathrm{H}}_2$ on the Si(001) surface

Adsorption geometries and vibrational modes of ${\mathrm{C}}_2{\mathrm{H}}_2$ on the Si(001) surface are studied by means of density-functional theory within the generalized gradient approximation. By comparing theoretically calculated high-resolution electron-energy-loss spectra with experimentally reported spectra, it is clearly shown that at least two energetically most stable adsorption states coexist.