Electronic Structure of the CO/Pt(111) Electrode Interface Probed by Potential-Dependent IR/Visible Double Resonance Sum Frequency Generation Spectroscopy

To clarify the origin of the anomalous increase of the intensity of the sum frequency generation (SFG) peak corresponding to the stretching of preadsorbed CO on a Pt(111) electrode in H₂SO₄ solution prior to anodic oxidation CO, potential-dependent SFG measurements were carried out by using eight different visible-light energies (527∼635 nm). Anomalous SFG intensity peaks were observed at all visible light except at 527 nm, and the potential of the SFG peak shifted with the visible-light energy by 1 V/eV, showing that the origin of the anomalous increase of SFG intensity is not due to the potential-dependent geometric structure change of adsorbed CO suggested before but due to a resonance of visible and/or SF light with electronic transition from the Fermi level of Pt(111) to the 5σ_a antibonding state of adsorbed CO. The present work showed that SFG spectroscopy can probe not only the molecular structure but also the electronic structure of the electrochemical interface, which is difficult to be determined by other methods.