Accurate determination of complex χ(2) spectrum of the air/water interface

Discussion on the structure of the water surface relies on accurate determination of the chi((2)) spectrum. For obtaining accurate chi((2)) spectrum of the air/water interface in the OH stretch region, we performed heterodyne-detected vibrational sum-frequency generation measurements with a high phase accuracy, and also examined the validity of the phase and amplitude calibration using different non-resonant materials. In contrast to the previous reports, it was concluded that the imaginary part of the chi((2)) spectrum of the air/water interface does not exhibit noticeable positive resonance in the low frequency region within the experimental error. This result urges us to reconsider the structure of the air/water interface based on the accurate chi((2)) spectrum. (C) 2015 AIP Publishing LLC.