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. Heterodyne-detected ESFG (HD-ESFG) spectroscopy allows us to acquire complex electronic χ
(2) spectra of interfacial solute molecules, and heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy enables us to obtain complex vibrational χ
(2) spectra of interfacial solvent and solute molecules. The complex χ
(2) spectra can directly indicate “up” versus “down” alignment of interfacial molecules. Frequency-domain χ
(4) Raman spectroscopy provides vibrational spectra of “buried” interfaces for a very wide wavenumber range covering the whole fingerprint region.
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