Chemical effects on vibrational properties of adsorbed molecules on metal surfaces: Coverage dependence

Abstract

Vibrational properties of chemisorbed molecules on metal surfaces are studied with a focus on the coverage dependent chemical shift of the frequencies. Available experimental data of a CO adsorption on transition metal and noble metal surfaces are analyzed in the light of the coverage dependent back-donation into the 2 π * orbitals of chemisorbed CO molecules. The vibrational frequency ω CO of the intramolecular stretching mode exhibits a downward shift of varying magnitude, depending on the amount of back-donation into the 2 π * orbitals of the chemisorbed CO. On increasing the coverage θ, ω CO usually increases due to the dipole-dipole interaction. On Cu surfaces, however, the shifts are relatively small, or in some cases, negative. So far, this anomalous frequency shift with θ is understood as a result of competitive effect between the upward dipole Ω dip and the downward chemical shift Ω chem associated with back-donation. The purpose of this paper is to establish the possible origin of the downward frequency shift through the electronic properties of an incomplete monolayer of adsorbates. The adsorbate density of states ρ a is calculated by means of the coherent potential approximation, in which the electron hopping between the adsorbates (band formation effect) and the depolarization effect due to the proximity of ionized adsorbed molecules are taken into account. The change of the occupied portion of ρ a and ρ a ( e F ) at the Fermi level e F with increasing θ then manifests itself in the coverage dependent Ω chem not only due to the static back-donation, but also due to the dynamical charge fluctuation during vibrational excitation. It is found that in a weakly chemisorbed system, such as CO/Cu, the negative Ω chem amounts to Ω dip at low θ . Consequently the apparent total frequency shift remains almost constant. As the coverage increases, Ω chem becomes larger than Ω dip due to the band effect. It is also shown that the variation of the back-donated charge with θ gives rise to the coverage dependent polarizability, which in turn influences the frequency shift estimated by the preiously dipole coupling theory. In the case of a strongly chemisorbed system, the molecules become negatively charged as a result of large back-donation. Depolarization fields, due to the increasing number of the adsorbed molecules carrying a negative charge, shift up the adsorbate energy level so that the occupation in ρ a decreases with θ . This leads to the positive Ω chem for strongly chemisorbed molecules, and explains why the coverage dependence of Ω CO of strongly chemisorbed CO on transition metal surfaces can be satisfactorily understood in terms of the dipole coupling theory. The coverage dependent back-donation also plays a significant role in the work function change Δ φ of the substrate upon CO adsorption. The polarity of a weakly chemisorbed CO molecule remains unchanged compared to free CO( − CO + ) so that Δ φ exhibits the initial lowering in the presence of the positive dipoles. The increase in the back-donated charge with θ causes the decrease in the effective dipole moment towards a compensation of the positive hole due to 5 σ donation. On the other hand, for a strongly chemisorbed CO, the polarity changes reversely as + CO − due to large back-donation, and Δ φ increases almost linearly up to a certain coverage where the sticking probability drops rapidly. A simple explanation is offered to clarify the characteristic difference of the work function change between the strongly and weakly chemisorbed CO molecules on metal surfaces. In particular, a possible origin of the work function minimum observed for CO/Cu systems is discussed in terms of the coverage dependent backdonation.