Surface-enhanced raman spectroscopy of electrochemically characterized interfaces. Relations between Raman scattering intensity and surface coverage for simple anionic adsorbates

Surface-enhanced raman scattering (SERS) obtained as a function of electrode potential for chloride, bromide, iodide, thiocyanate, azide, and cyanide adsorbed at roughened silver electrodes is compared with corresponding surface concentration-potential data extracted from differential capacitance measurements in order to examine the relation between SERS and surface coverage for these structurally simple adsorbates. After generating SERS by means of an oxidation-reduction cycle, it was found that altering the potential to a more negative value, where the adsorbate coverage fell below a monolayer, corresponded closely in most cases to the onset of a potential-dependent decay in the SERS intensity. Monitoring the potential dependence of the Raman intensity with an optical multichannel analyzer as well as with a conventional scanning spectrometer allowed a rapid "reversible" component of the potential dependence to be separated from an additional "irreversible" signal decay associated with the loss of Raman-active sites. Examination of potential-dependent SERS sufficiently rapidly so that reversible conditions prevail has the crucial advantage of holding constant the concentration of Ramanactive sites. For adsorbed chloride and bromide, an approximate correlation was found between the fractional coverage and the variation in the corresponding reversible Raman intensity brought about by altering the electrode potential. The present results suggest that the adsorption energetics of the Raman-active surface sites do not differ substantially from those for the sites occupied by the majority of the adsorbate.