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The uniqueness of water originates from its three-dimensional hydrogen-bond network, but this hydrogen-bond network is suddenly truncated at the interface and non-hydrogen-bonded OH (free OH) appears. Although this free OH is the most characteristic feature of interfacial water, the molecular-level understanding of its dynamic property is still limited due to the technical difficulty. We study ultrafast vibrational relaxation dynamics of the free OH at the air/water interface using time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. With the use of singular value decomposition (SVD) analysis, the vibrational relaxation ( T  1 ) times of the free OH at the neat H 2 O and isotopically-diluted water interfaces are determined to be 0.87 ± 0.06 ps (neat H 2 O), 0.84 ± 0.09 ps (H 2 O/HOD/D 2 O = 1/2/1), and 0.88 ± 0.16 ps (H 2 O/HOD/D 2 O = 1/8/16). The absence of the isotope effect on the  T  1  time indicates that the main mechanism of the vibrational relaxation of the free OH is reorientation of the topmost water molecules. The determined sub-picosecond  T  1  time also suggests that the free OH reorients diffusively without the switching of the hydrogen-bond partner by the topmost water molecule.

Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy