FEMTOSECOND MID-IR PUMP-PROBE SPECTROSCOPY OF LIQUID WATER - EVIDENCEFOR A 2-COMPONENT STRUCTURE

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