Vibrational modes of CO adsorbed on disordered copper films

Abstract 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 σ‐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.