First principles calculations of the vibrational dynamics of c(2x2)-CO on Ag(001).*

The reaction pathway of CO oxidation on Ag surfaces is still a subject of debate because of the complicated chemistry of O and the possibility that contaminants stabilize CO. Indeed, at $\sim $150 K, the dissociative O$_{2}$ adsorption is scarcely triggered while the CO adsorption on clean Ag(001) is hardly stable. The nature of the CO adsorption is thus by itself a matter of discussion and the characteristic energy losses for exciting the phonon modes introduced by the adsorbed species have an uncertain assignment. We present an \textit{ab initio} study of the structure and phonon dispersion of a c(2x2), atop, CO overlayer on Ag(001). Comparison with a similar study of c(2x2) CO on Cu(001) indicates that CO chemisorbs on Ag(001) despite the low binding energy. The frequency of the C-O stretch mode at the $\Gamma $-point is in excellent agreement with HREELS measurements and is reduced on Ag(001) almost as much as on Cu(001). The weak Ag-CO bond is reflected in the low frequency of the rest of the CO modes. Yet, in the Ag-CO stretch, the Ag surface atoms are strongly coupled, as in the case of CO on Cu(001). Likewise, the CO frustrated translation mode couples to the substrate in the vicinity of the $\Gamma $-point but, unlike that on Cu(001), the CO frustrated rotation mode on Ag(001) couples to the substrate inside the surface Brillouin zone.