Surface enhanced Raman scattering as a probe of adsorbate-substrate charge-transfer excitations

The chemical mechanism of surface enhanced Raman scattering (SERS) is used as a probe of adsorbate-substrate charge-transfer excitations on atomically smooth, single crystal copper surfaces in ultra-high vacuum. It is demonstrated that SERS and charge-transfer excitations are supported on atomically smooth surfaces in contradiction to earlier consensus. The combination of SERS with electron energy loss spectroscopy (EELS) provides a great deal of insight into the nature of these excitations. It is found that these excitations are sensitive probes of local electronic structure at metal surfaces. The spatial extent, transition moment, and excited state potential energy surfaces of these excitations are probed using SERS, EELS and molecular spectroscopy simulations. (C) 1999 Elsevier Science B.V. All rights reserved.