Bend Vibration of Surface Water Investigated by Heterodyne-Detected Sum Frequency Generation and Theoretical Study: Dominant Role of Quadrupole

Heterodyne-detected vibrational sum frequency generation spectroscopy was applied to the water surface for measuring the imaginary part of second-order nonlinear susceptibility (Im χ⁽²⁾) spectrum in the bend frequency region for the first time. The observed Im χ⁽²⁾ spectrum shows an overall positive band around 1650 cm^-1, contradicting former theoretical predictions. We further found that the Im χ⁽²⁾ spectrum of NaI aqueous solution exhibits an even larger positive band, which is apparently contrary to the flip-flop orientation of surface water. These unexpected observations are elucidated by calculating quadrupole contributions beyond the conventional dipole approximation. It is indicated that the Im χ⁽²⁾ spectrum in the bend region has a large quadrupole contribution from the bulk water.