Absolute vibrational spectra of the electrochemical H/Pt and H/Pt(100) interfaces in the under- and overpotential ranges are measured by ir-vis sum frequency generation. The vibrational signature of the underpotential-deposited hydrogen is structure sensitive and is related to its bonding configuration and to the water molecules located in the double layer region. A new hydrogen adspecies, presumed to be the intermediate for the hydrogen evolution reaction, shows up in the overpotential range, with a similar vibrational fingerprint on both surfaces.

• Received 10 May 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.3010