Frequency-Dependent Vibrational Relaxation Time of OH Stretch at the Air/Isotopically Diluted Water Interface.

Elucidation of the vibrational relaxation process of interfacial water is indispensable for understanding energy dissipation at the aqueous interface. In this study, the vibrational relaxation dynamics of the hydrogen-bonded OH (HB OH) stretch vibration was investigated at the air/isotopically diluted water (HOD-DO) interface by time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. We observed the temporal change of the excited-state band ( = 1 → 2 transition), which enables a reliable determination of the time of interfacial water. The times obtained for the HB OH stretch with various pump frequencies are 0.14 ± 0.15 ps (3200 cm), 0.27 ± 0.05 ps (3300 cm), 0.34 ± 0.03 ps (3400 cm), and 0.63 ± 0.04 ps (3500 cm), indicating that is comparable with the value at the air/HO interface at the low-frequency side but is markedly longer at the high-frequency side. The observed frequency-dependent time can be rationalized in terms of the frequency mismatch between the HB OH stretch and the bending overtone in HOD-DO, supporting the conclusion that vibrational relaxation through the Fermi resonance with the bending overtone is the predominant mechanism of the vibrational relaxation of the HB OH stretch at the air/water interface.