Surface Enhancement Factors for Raman Scattering at Silver Electrodes. Role of Adsorbate-Surface Interactions and Electronic Structure.
PURDUE UNIV LAFAYETTE IN DEPT OF CHEMISTRY
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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 CrIII complexes, and also for CrIII and CoIII hexammines 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 pyrazinepentaammineosmiumIII and II to explore the influence of electronic resonance since the OsII form displays an intense charge-transfer band in the vicinity of the laser wavelengths 460-650 nm employed. A pronounced peak in the OsII excitation profile, absent for OsIII, is obtained that is markedly ca. 100 nm red shifted for the adsorbed versus bulk complex.
- Atomic and Molecular Physics and Spectroscopy