Molecular reorientation in hydrogen-bonding liquids: through algebraic $\sim t^{-3/2}$ relaxation toward exponential decay

Abstract

We present a model for the description of orientational relaxation in hydrogen-bonding liquids. The model contains two relaxation parameters which regulate the intensity and efficiency of dissipation, as well as the memory function which is responsible for the short-time relaxation effects. It is shown that the librational portion of the orientational relaxation is described by an algebraic $\sim t^{-3/2}$ contribution, on top of which more rapid and non-monotonous decays caused by the memory effects are superimposed. The long-time behavior of the orientational relaxation is exponential, although non-diffusional. It is governed by the rotational energy relaxation. We apply the model to interpret recent molecular dynamic simulations and polarization pump-probe experiments on $HOD$ in liquid $D_{2}O$ [C. J. Fecko et al, J. Chem. Phys. 122, 054506 (2005)].