Influence of foreign metal atoms deposited at electrodes on local and nonlocal processes in Surface Enhanced Raman Scattering

Surface Enhanced Raman Scattering (SERS) from electrodes exhibits a reversible potential dependence in a limited potential range. Cathodic of this range it is irreversibly quenched on Ag electrodes by more than one order of magnitude. This holds for adsorbed pyridine molecules, for chloride, bromide, iodide, cyanide or thiocyanate ions, and for water or deuterium oxide. Since the (sub)microscope roughness is not altered by this procedure, the quenching has to be explained by a vanishing of SERS active sites, an idea which is clearly confirmed in metal deposition experiments using SERS for pyridine as a probe. In some cases, the deposition of a fraction of a monolayer (θ ca. 0.01) of a foreign metal on Ag is sufficient to significantly modify the intensity, the Raman shift and the halfwidth of the SERS bands. For example, the almost total quenching of SERS on Ag electrodes at potentials cathodic of ?1.7V is prevented, to a great extent, by Cu coverages of θ=0.003. This evidences a low density of active sites. In other cases the impurity metal affect mainly on the SERS intensity, obviously by damping the electromagnetic resonances. These effects clearly reveal the importance and cooperation of local and non-local enhancement processes.