Vibrational spectroscopy of the electrochemical interface by visible infrared sum frequency generation

Vibrational properties of species adsorbed on platinum electrodes in the electrochemical environment are investigated by visible-infrared sum-frequency generation (SFG). Absolute vibrational spectra of adsorbed species are obtained by combining the inherent interfacial selectivity of SFG between centrosymmetric media and the use of a free-electron laser as infrared source. SFG spectra of hydrogen on polycrystal and single crystal platinum electrodes in acid medium are measured both in the under- and in the overpotential region. The vibrational fingerprints of the underpotential deposited hydrogen are structure sensitive and are related to its bonding to interfacial water molecules. On Pt(111) electrode, we have detected adsorbed hydrogen at potential as high as 0.35 V, within the so-called unusual adsorption state. A new hydrogen adspecies shows up in the overpotential range. Its vibrational signature is not dependent on the electrode surface orientation and we have assigned it to the intermediate of hydrogen evolution.