A unified treatment of the dipole and quadrupole contributions to the second-order optical response is developed. The concept of the effective susceptibility is expanded to incorporate both the surface and bulk contributions. This treatment allows us to quantitatively evaluate both surface and bulk contributions in experimental SFG/SHG spectra. The present theory clarifies a number of qualitative features in the quadrupole contribution, and is thereby utilized to settle recent confusions about the quadrupole and/or bulk effects on the surface nonlinear spectroscopy.
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