Nuclear Quantum Effect on the χ(2) Band Shape of Vibrational Sum Frequency Generation Spectra of Normal and Deuterated Water Surfaces.

In the vibrational sum frequency generation (VSFG) spectrum of the air/water interface, there is an open question whether the imaginary spectrum of nonlinear susceptibility Im[χ(2)] has a positive band in the low-frequency tail of the OH stretching band. We previously elucidated by molecular dynamics (MD) analysis of the VSFG spectrum that the positive band could arise from particularly strong hydrogen-bonded water pairs at the water surface. This mechanism should be emphasized in OD stretching in comparison to OH, because OD forms stronger hydrogen bonds. Therefore, we calculated the Im[χ(2)] spectra of normal and deuterated water surfaces by MD simulation including the nuclear quantum effect and demonstrated that the low-frequency positive feature could arise in the tail of the OD stretching band. This positive feature is sensitive to the oxygen-oxygen distance of hydrogen-bonding pairs at the surface and hence the temperature and disappears with increasing temperature.