Abstract
The reorientational motion of the molecules in liquid water is investigated by measuring the anisotropy decay of the O–H stretching mode of HDO dissolved in D 2O using femtosecond mid-infrared pump–probe spectroscopy. We observe that the anisotropy shows a non-exponential decay with an initial fast component of which the amplitude increases with increasing lengths of the O–H⋯O hydrogen bond. The experimental results can be accurately described with a model in which the dependence of the reorientation rate on the hydrogen-bond length and the stochastic modulation of this length are accounted for. It is found that the O–H group of a water molecule can only reorient after the O–H⋯O hydrogen bond has sufficiently lengthened. As a result, the effective rate of reorientation of the molecules in liquid water is determined by the rate at which the length of the hydrogen bonds is modulated.
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