Enhancement of the dynamical dipole moment in adsorbed molecules: Theoretical studies for CO/Cu(100)

Ab initio cluster-model calculations have been performed for the C-O stretch of CO/Cu(100). Attention has been given to the dynamical dipole moment which is decomposed into contributions from the $\sigma$ and $\pi$ electrons of the cluster. The largest contribution is from the $\pi$ electrons, and the increase in the value of this contribution for CO/Cu(100) over that for free CO is due, mainly, to the metal to CO $2{\pi }^{*}$ donation. Our theoretical value for the enhancement of the dynamical dipole moment for adsorbed CO is consistent with results of recent experiments. For the first time, we present direct ab initio evidence for the important role which the metal-to-CO $2{\pi }^{*}$ donation plays for this enhancement.