Investigation of the Quenching of Surface Enhanced Ranian Scattering from Pyridine on Copper and Silver Electrodes by Underpotential Deposition of Lead

Experiments designed to investigate the effect of the underpotential deposition (UPD) of Pb on the surface enhanced Raman scattering (SERS) of pyridine adsorbed on Cu electrodes are described. Drawing on the wealth of information published on the SERS of Ag, the approach taken in these studies is to directly compare the SERS response of Cu to Ag under similar conditions. An excitation study of the SERS quenching of pyridine adsorbed at these electrodes by Pb UPD is presented. The quenching of Cu SERS at different excitation energies does not emulate the previously reported Ag response. Both electromagnetic (EM) and charge transfer (CT) mechanisms are considered in interpreting this behavior. Optical property measurements as well as EM enhancement calculations based on a model described by Murray for Pb submonolayer films on Cu and Ag are discussed. The EM predicted quenching behavior is inadequate in explaining the experimental results. The CT predictions, however, correlate well to the results. Further investigation of the quenching differences of the two systems through negative potential excursion studies reveals an increased stability of the Cu CT sites relative to those of Ag. The differences observed in the quenching profiles of the two systems are attributed to the increased stability of the Cu CT sites.