Because of strong hydrogen bonding in liquid water, intermolecular interactions between water molecules are highly delocalized. Previous two-dimensional infrared spectroscopy experiments have indicated that this delocalization smears out the structural heterogeneity of neat H2O. Here we report on a systematic investigation of the ultrafast vibrational relaxation of bulk and interfacial water using time-resolved infrared and sum-frequency generation spectroscopies. These experiments reveal a remarkably strong dependence of the vibrational relaxation time on the frequency of the OH stretching vibration of liquid water in the bulk and at the air/water interface. For bulk water, the vibrational relaxation time increases continuously from 250 to 550 fs when the frequency is increased from 3,100 to 3,700 cm-1. For hydrogen-bonded water at the air/water interface, the frequency dependence is even stronger. These results directly demonstrate that liquid water possesses substantial structural heterogeneity, both in the bulk and at the surface.

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Journal Nature Commun.
van der Post, S.T, Hsieh, C-S, Okuno, Masanari, Nagata, Y, Bakker, H.J, Bonn, M, & Hunger, J. (2015). Strong frequency dependence of vibrational relaxation in bulk and surface water reveals sub-picosecond structural heterogeneity. Nature Commun., 6(Article number: 8384), 1–7. doi:10.1038/ncomms9384