A deep understanding of electrochemical processes generally requires a combination of electrochemical and spectroscopic methods. While electrochemical analysis can provide the potential, current and charge information, the spectroscopies can reveal the electronic and vibrational properties of the reaction intermediates or products. The combination is referred to as spectro-electrochemistry.
Among the modern spectro-electrochemical techniques, vibrational spectroscopy such as Infrared absorption and Raman scattering are powerful in terms of providing structural and chemical information of interfacial species and have thus been widely employed for in situ spectro-electrochemical characterization. However, IR and Raman spectroscopies are not surface specific. When the surface and the bulk have similar chemical compositions, i.e. bulk solvents and anions adsorb to the electrode surface, it is very challenging to selectively probe the adsorbed species and discriminate from those of the bulk. Vibrational sum frequency spectroscopy (VSFS), based on the 2nd order nonlinear optical process, is intrinsically surface specific hence an excellent tool in probing buried interfaces.