On the role of charge-transfer resonances in the chemical mechanism of surface-enhanced Raman scattering

The surface-enhanced Raman spectrum of pyromellitic dianhydride (PMDA) on Cu(100) is reported; it is different from that previously reported for the Cu(lll) surface. The spectra are sensitive to the incident laser polarization on both surfaces but in different ways. These observations are shown to be inconsistent with a simple classical picture which considers only the interaction of the surface electromagnetic fields with the anisotropic molecular polarizability. They lend additional support to the hypothesis that the chemical enhancement mechanism involves resonance-Raman scattering via a charge-transfer intermediate state. Simulations suggest that the scattering is primarily Franck-Condon and Jahn-Teller in origin.