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Computational analysis of the quadrupole contribution in the second-harmonic generation spectroscopy for the water/vapor interface.

Second-order susceptibility for water/vapor interface is calculated theoretically using molecular dynamics simulation, which considers both the dipole and quadrupole contributions. We find that the nonresonant second harmonic generation (SHG) signal is dominated by the quadrupole contribution from the bulk. We also elucidate the fact that the nonresonant susceptibility tends to be negative in general, irrespective of the molecular orientation. The present argument for SHG is commonly applicable to the nonresonant vibrational sum-frequency generation (SFG) spectra, and should be valid for other liquid interfaces.