Surface enhancement factors for Raman scattering at silver electrodes. Role of adsorbate–surface interactions and electronic structure

Abstract

The integrated Raman band intensities for internal vibrational modes of various adsorbates at roughened silver electrodes are combined with surface concentrations obtained electrochemically to yield surface Raman scattering cross sections. These data together with the corresponding bulk-phase Raman intensities provide estimates of the degree of enhancement of the vibrational bands at the silver surface with respect to the bulk medium, i.e., the ‘‘surface enhancement factors,’’ SEF. For surface-attached thiocyanate, isothiocyanate-bridged Cr(III) complexes, and also for Cr(III) and Co(III) hexaammines that are electrostatically attracted to chloride-coated silver, SEF values are obtained that are relatively insensitive to both the vibrational mode and the adsorbate, even though substantial (up to 500 fold) variations are observed in the Raman cross sections. This finding applies to stretching modes involving nonbridging ammine groups as well as to bridging thiocyanate modes, although the frequencies of the latter, but not of the former, are perturbed strongly by the silver surface. Comparisons were also made between Raman cross sections, surface enhancement factors, and excitation profiles for pyrazinepentaammineosmium(III) and (II) to explore the influence of electronic resonance since the Os(II) form displays an intense charge-transfer band in the vicinity of the laser wavelengths (460–650 nm) employed. A pronounced peak in the Os(II) excitation profile, absent for Os(III), is obtained that is markedly (∼100 nm) red shifted for the adsorbed vs bulk complex.