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 la ser polarization on both surfaces but in different ways. These observa tions are shown to be inconsistent with a simple classical picture whi ch considers only the interaction of the surface electromagnetic field s with the anisotropic molecular polarizability. They lend additional support to the hypothesis that the chemical enhancement mechanism invo lves resonance-Raman scattering via a charge-transfer intermediate sta te. Simulations suggest that the scattering is primarily Franck-Condon and Jahn-Teller in origin.