To understand the mechanism and improve the efficiency of electrochemical processes at solid/liquid interfaces, it is essential to have in situ, real time information of geometric, electronic, and molecular structures of the solid/liquid interfaces with atomic and molecular resolution. However, many techniques applicable to vacuum condition utilizing electrons as a probe cannot be applied to solid/liquid interfaces because of the presence of the liquid. The determination of electronic structures is particularly difficult. Here we show that sum frequency generation (SFG) spectroscopy, which is generally used to obtain interfacial molecular structure by utilizing IR resonance to the interfacial vibrational transition, can be used to determine the electronic structure of electrochemical interfaces by utilizing visible resonance to the interfacial electronic transition with the potential control of Fermi level. Potential dependent SFG measurements were carried out at a Pt(111) electrode with pre-adsrobed CO in H2SO4 solution by using visible light of various energies. SFG peak corresponding to CO stretching of CO adsorbed on the atop site of Pt(111) was observed and its intensity anomalously increased prior to CO oxidation. The potential, at which the SFG intensity became maximum, depended on the energy of visible light, showing that the origin of the anomalous increase of SFG intensity is due to a resonance of visible and/or SF light with electronic transition from the Fermi level of Pt(111) to the 5σa anti-bonding state of adsorbed CO. The present work showed that SFG spectroscopy can probe not only the molecular structure but also the electronic structure of electrochemical interface.

Potential dependent IR/visible double resonance sum frequency generation spectroscopy to probe electronic structure at electrochemical interfaces

251st National Meeting (2016)

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