VIBRATIONAL-MODES OF CO ADSORBED ON DISORDERED COPPER-FILMS

The Raman spectrum of CO adsorbed on copper films featuring atomic-scale roughness reveals four vibrational modes: a CO stretching mode at 2102 cm-1, a restricted rotation at 282 cm-1 and restricted translations normal and parallel to the local surface at 355 and 24 cm-1. The spectrum originates from a minority of CO molecules adsorbed at surface defect sites via a single coordination bond. The bond strength is increased compared with adsorption at facet sites, presumably owing to increased sigma-donation promoted by a local depletion of conduction electron density. A splitting of the translational mode into two separate frequencies, expected for adsorption sites with rotational symmetry less than C(3v), is not observed. The intensities and line shapes of the SERS bands are sensitive to a thermally induced rearrangement of the molecules within the adsorbate layer. Spectra from annealed substrate films indicate dominant excitation of the rotational mode on surfaces with a lesser degree of atomic-scale roughness. This is explained by the relevance of Herzberg-Teller contributions to the inelastic scattering of photoexcited electrons by the surface molecules.