Surface-enhanced resonance Raman scattering (SERRS) spectra were measured at 0 V (vs Ag/AgCI) for a hemicyanine dye and its protonated form (HC and HCH') adsorbed on a silver electrode surface. The surface spectrum of HC shows a characteristic feature due to a trans conformation, a band mainly due to a stretching frequency ( v~=~) at 1580 cm-', which is appreciably lower than the corresponding frequency for HC in water (1592 cm-I); this result indicates that, on adsorption on the electrode surface, the resonance balance of HC between the quinoid and benzenoid forms shifts to the former. An adsorbate on a silver electrode surface from HCH' solution at pH 1.0 takes either a protonated form, HCH', or a deprotonated form, HC, depending on the anodization conditions of the electrode surface. A scanning electron microscopic study indicated that the adsorbate on a rough surface consisting of granular structures assumes the deprotonated form, whereas the adsorbate on a surface containing smooth parts retains the protonated form; hence the surface morphology is related to the chemical properties of the silver electrode surfaces. It was also found that the adsorbate in the protonated state gives a prominent SERRS band at 885 cm-' assignable to an out-of-plane bending vibration of the central olefinic group, indicating that the adsorbate assumes a flat orientation on the silver electrode surface.

Time-resolved resonance Raman and surface-enhanced resonance Raman scattering (RRS and SERRS) spectra were measured for the monocation radical of heptylviologen (HV'+) formed on a silver electrode surface as a function of time ( t ) after switching the electrode potential from -0.2 V (vs Ag/AgCl) to -0.65 V (the monocation radical formation potential). The intensity of the R R S band at 1530 cm-I due to HV'+ increases as a linear function of t 3 / 2 in the first step (0 to ca. 4 ms) and as a linear function of t l / * in the second (4-10 ms) and third steps (10-30 ms). The t312-dependence indicates an instantaneous nucleation mechanism, which consists of instantaneous formation of a nucleus and subsequent three-dimensional growth of the nuclei, and the t'i2-dependence indicates one-dimensional diffusion-controlled film growth processes. Measurement of transient currents confirmed the existence of the two kinds of one-dimensional diffusion-controlled film growth processes. Time-resolved SERRS spectral measurements proved that the monocation radical formation process in a few monolayers of the HV2+ molecules adsorbed at a roughened silver electrode surface proceeds following a mechanism similar to that of the first step observed for the smooth silver electrode surface. The time-resolved R R S and SERRS spectra indicated that at the beginning of the reduction process the radicals exist as a dimeric state at both the smooth and roughened silver electrode surfaces and that the conversion from the dimer to a radical salt begins when the radicals form a film of a thickness of at least 5 monolayers.

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