Observation of surface-enhanced Raman scattering for transition-metal hexaammine cations at the outer Helmholtz plane: Implications for enhancement mechanisms at electrochemical interfaces

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Abstract
Surface–enhanced Raman scattering (SERS) has been observed at silver electrodes for several transition‐metal hexaammine complexes even though these cations cannot bind to the metal surface and are excluded from the electrochemical inner layer by the presence of a monolayer of chloride or bromide anions. The intensity of the SERS metal‐ammine stretching vibrations νM−N are comparable to those seen for closely related pentammine complexes that are bound to the surface via specifically adsorbed coordinated ligands. For hexammineruthenium(III), changes in the νM−N frequency with potential indicate the presence of an interfacial Ru(NH3)3+/2+6 couple. The formal potential of this couple obtained from the potential dependence of the SERS intensities is consistent with the Raman scattering ions being located at the outer Helmholtz plane. The implications of these results to current models of SERS in electrochemical environments are noted.