VIBRATIONAL SPECTROSCOPY OF THE ELECTROCHEMICAL INTERFACE BY VISIBLE-INFRARED SUM-FREQUENCY GENERATION

Vibrational properties of species adsorbed at the electrochemical interface are investigated by visible-infrared sum-frequency generation (SFG). Absolute vibrational spectra are obtained even for systems like H-pt with a very weak IR cross-section by combining the inherent interfacial selectivity of SFG between centrosymmetric media and the use of a free electron laser (FEL) as IR source. The potential dependence of SFG spectra of CN- on silver electrode in contact with (0.1M NaClO4 + 0.01M KCN) solution gives access to the determination of the potential of zero charge and the adsorption behaviour of CN- on Ag. The SFG resonance spectra of intermediate species produced by the dissociative adsorption of methanol in Pt-HClO4 interface were recorded as a function of the electrode potential and the methanol concentration. Two resonances associated with the linearly and multi-bound CO have been identified. In contrast to previous IR studies, our results reveal a comparable occupation of both adsorption sites at high methanol concentration. Moreover the relative occupation of both sites strongly depends on methanol concentration, indicative of a dynamic equilibrium between species in solution and in adsorbed phase. Finally absolute vibrational spectra of hydrogen on a polycrystal and a single crystal platinum electrode in acid medium are measured both in the under and in the overpotential region. The vibrational fingerprints of the underpotential deposited hydrogen is structure sensitive and is related to its bonding to interfacial water molecules. A new hydrogen adspecies assumed to be the intermediate for the hydrogen evolution reaction shows up in the overpotential range. Its vibrational signature is not sensitive to the electrode structure.