Novel interface‐selective even‐order nonlinear spectroscopy

New interface‐selective second‐ (χ(2)) and fourth‐order (χ(4)) nonlinear spectroscopic techniques have been developed. χ(2) electronic sum frequency generation (ESFG) spectroscopy enables us to obtain interfacial electronic spectra with an unprecedented high signal‐to‐noise ratio and dense wavelength data points. Frequency‐domain χ(4) Raman spectroscopy provides vibrational spectra of interfaces for a very wide wavenumber range covering the whole fingerprint region. Because these new even‐order electronic and vibrational nonlinear spectroscopies utilize only visible and/or near‐infrared laser pulses, they are applicable to a variety of “buried” interfaces that are not readily accessed by the other existing methods.