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Frequency-Dependent Vibrational Relaxation Time of OH Stretch at the Air/Isotopically Diluted Water Interface
Erika Kinoshita1,2, Woongmo Sung1, Satoshi Nihonyanagi1,3
1Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan.
Abstract:
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-D2O) interface by time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. We observed the temporal change of the excited-state band (v = 1 → 2 transition), which enables a reliable determination of the T1 time of interfacial water. The T1 times obtained for the HB OH stretch with various pump frequencies are 0.14 ± 0.15 ps (3200 cm-1), 0.27 ± 0.05 ps (3300 cm-1), 0.34 ± 0.03 ps (3400 cm-1), and 0.63 ± 0.04 ps (3500 cm-1), indicating that T1 is comparable with the value at the air/H2O interface at the low-frequency side but is markedly longer at the high-frequency side. The observed frequency-dependent T1 time can be rationalized in terms of the frequency mismatch between the HB OH stretch and the bending overtone in HOD-D2O, 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.
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