Bonding and Vibrational Properties of CO-adsorbed Copper

Accurate density functional calculations are performed to investigate the structure and vibrational dynamics of carbon monoxide adsorbed to the (100) surface of copper. The adsorbate and substrate are considered as a unified system, with atoms of each treated on an equal footing. Coupling between the two components is found to have a significant effect. In particular, frustrated translational motion mixes strongly with transverse phonons of the substrate to form a broad resonance. Direct computation of anharmonic coupling between the internal CO bond stretching mode and other adsorbate-weighted modes confirms the experimental conclusion that the transient CO-stretch response seen in recent pump-probe studies is an indirect probe of the transient dynamics of frustrated translations. In this light, the computed resonance between this mode and substrate phonons suggests a dephasing mechanism to account for the observed relaxation dynamics.