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.