Femtosecond mid-IR pump-probe spectroscopy of liquid water: Evidence for a two-component structure

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 t_R = 13 picoseconds and t_R = 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.