Cluster-Model Calculation of Raman Intensity for Vibration of CO Adsorbed on Copper.

The discovery of surface enhanced Raman spectroscopy has inspired a large body of work which has been summarized in several recent review articles. While much has been learned about various enhancement sources there is still disagreement regarding the proportion in which different factors contribute to the observed Raman cross section. Even though it is difficult to include all the opinions in a broad classification, it is fair to say that the main debate is centered around the relative importance of chemical versus electromagnetic (EM) enhancement factors. As emphasized in the Introduction, the intention was to estimate to what extent the non-resonant Raman signal for the C-O stretch is increased when CO binds to a Cu surface. An HCUCO cluster has been used to simulate the on-top binding of CO on Cu(100). The fact that our cluster reproduces the binding properties of adsorbed CO and the CO valence orbitals of pyramidal Cu5CO gives us hope that a correct estimate of the Raman intensity has been obtained. This is especially likely since the property sought is a local quantity. Our result is that the chemical enhancement is about a factor of four which is in general agreement with the few careful estimates based on experimental data. We emphasize that the present calculation does not deal with the question of resonant enhancement.