Quadrupole Contribution of C═O Vibrational Band in Sum Frequency Generation Spectra of Organic Carbonates.

Sum Frequency Generation (SFG) is usually governed by surface-selective signals of dipole origin, but it can also contain some bulk signals of quadrupole origin. In this work, we examined the dipole and quadrupole contributions in the C═O stretching band of organic carbonate liquids with collaboration of heterodyne SFG measurement and theoretical analysis. As a result, we found that these spectra are substantially affected by the quadrupole contribution of the bulk, which resolved the discrepancy between the experimental and computational SFG spectra.