A femtosecond mid-infrared pump-probe study of the vibrational and orientational dynamics of the OH-stretching mode of HDO dissolved in D2O is presented. The orientational relaxation of the HDO molecules was observed to occur on either a very slow or a very fast time scale, with associated time constants of tR = 13 picoseconds and tR = 0.7 picosecond. It was observed that strongly hydrogen-bonded water molecules only relax through the slow orientational relaxation process, whereas the fast process dominates for weakly hydrogen-bonded molecules. This suggests that, with respect to orientional dynamics, two distinct molecular species exist in liquid water.

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Journal Science
Woutersen, S, Emmerichs, U, & Bakker, H.J. (1997). Femtosecond mid-IR pump-probe spectroscopy of liquid water: Evidence for a two-component structure. Science, 278, 658–660.