Contributions of Atomic-Scale Roughness and Adsorbate Coverage to the Quenching of the SERS Response at Lead-Modified Silver Electrodes

This report details a series of experiments and calculations designed to provide a more complete analysis of the factors which may contribute to the quenching of the SERS response at Pb-modified Ag electrodes. SERS intensity-Pb coverage profiles are presented for the pyridine ring-breathing vibration after the majority of atomic-scale roughness features are destroyed by Pb monolayer deposition. Comparison of the morphology of the intensity-coverage profiles obtained during and after the loss of atomic-scale roughness indicates that loss of the majority of atomic-scale roughness during Pb deposition cannot fully account for the observed quenching of the SERS response. The SERS response of 3,6-dihydroxypyridazine (DHPZN) is examined in order to evaluate the impact of adsorbate coverage on the observed quenching behavior. X-ray photoelectron spectroscopic studies of DHPZN adsorbed at Ag and Pb-modified Ag electrodes indicate that changes in the adsorbate surface coverage during Pb deposition do not contribute significantly to the quenching of the SERS response. Theoretical SERS enhancement-Pb coverage profiles based on two different electromagnetic predictions and previously calculated optical constants for Pb-modified Ag surfaces are presented. The functional form of the linear approximation expression used for evaluation of surface optical constants from differential reflectivity data is examined in order to provide further insight into the role of surface electronic properties in the SERS mechanism(s).