SURFACE-ENHANCED RESONANCE RAMAN-SCATTERING FROM METHYLVIOLOGEN AT A SILVER ELECTRODE - EVIDENCE FOR 2 DISTINCT ADSORPTION INTERACTIONS

The electronic absorption and resonance Raman spectra of methylviologen radical cation (MV•+) and fully reduced methylviologen (MV0) have been characterized. The enhancement of Raman and resonance Raman (RR) scattering from the dication and its reduction products at polished and roughened silver electrodes was also investigated. The spectrum of MV•+ on a polished silver electrode, denoted as type I, is enhanced by a factor of approximately 300 with 457.9-nm excitation. The spectrum is very similar to that of the cation radical in solution. In contrast, the spectrum observed on an oxidation-reduction cycle roughened Ag electrode, denoted as type II, is radically different with respect to band frequencies, intensities, and potential dependence from that obtained on the polished electrode and from solution. Conditions for observing the two types of spectra are described. It is concluded that type I and type II spectra result from electromagnetic and chemical enhancement, respectively. © 1990 American Chemical Society.