Femtosecond Mid-IR Pump-Probe Spectroscopy of Liquid Water: Evidence for a Two-Component Structure

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Science  24 Oct 1997:
Vol. 278, Issue 5338, pp. 658-660
DOI: 10.1126/science.278.5338.658

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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 τR = 13 picoseconds and τR = 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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