Ab initio calculations of the dispersion of surface phonons of ac(2 × 2) CO overlayer on Ag(001)

Abstract

We examine the phonon dispersion ofc(2 × 2)-CO on Ag(001) by applying density functional perturbation theory withthe generalized-gradient approximation. Our calculations indicate that thec(2 × 2)-CO overlayer on Ag(001) is dynamically stable. We find thatthe bond length of CO is expanded and its stretch mode (ν1) softenedby ∼ 9 meV upon adsorption on Ag(001), in excellent agreement with experiments. We show thatν1 at alone cannot gauge the metal–CO interaction since it is not entirelydetermined by the C–O intramolecular force constant. Further softening ofν1 on Ag(001) is obtained outside , indicative of CO–CO interactions even at a distance of ∼ 4 Å. The frequency of the Ag–CO stretch mode (ν2) is ∼ 30 meV and it is nearly dispersionless, implying that the perturbation correspondingto this mode is short-ranged. The frustrated rotation mode of CO (ν3) overlaps with the bulk band and mixes with substrate modes inside the SBZ,suggesting this as one of the key features for the enhanced diffusivity of CO onAg surfaces over that on Cu surfaces. The frustrated translation mode of CO (ν4) is everywherebelow ∼ 2.8 meV and therefore mixes with substrate modes in the region of the SBZ around . Depending on the q vector, vertical and in-plane surface modes may soften or stiffen with respect to their counterparton clean Ag(001). Although the response of most Ag(001) modes to CO adsorption is similarto that of corresponding Cu(001) modes, there are some contrasting features between thedynamics of the two surfaces concerning the changes in the surface force constants and mixing ofS6 withν3 on Ag(001).