First-principles molecular dynamics study of CO adsorption on the Si(001) surface

We present first-principles molecular dynamics calculations of CO adsorption on the Si(001) surface. We found two adsorption sites of CO on the Si(001) surface, on which the CO molecule adsorbs symmetrically and asymmetrically. The asymmetric structure is preferable: the calculated adsorption energies are 17.0 and 19.0 kcal/mol, respectively, on the symmetric and asymmetric sites. The electron density analysis shows that the main interaction between CO and the Si(001) surface is σ-donation on the asymmetric sites and π-back-donation on the symmetric sites. The calculated vibrational frequency of the CO stretch for the asymmetric site agrees well with the experimental HREELS spectra. The symmetric structure may also exist considering the small difference in the adsorption energy.