Femtosecond Mid-infrared Study of the Reorientation of Weakly Hydrogen-bonded Water Molecules

We study the hydrogen-bond and reorientation dynamics of weakly hydrogen-bonded water molecules by studying their spectral diffusion and anisotropy dynamics with polarization-resolved two-color femtosecond mid-infrared spectroscopy. We selectively excite weakly hydrogen-bonded water molecules by tuning a relatively narrow band excitation pulse far into the high-frequency wing of the O–D stretch vibration of HDO molecules in H₂O water. We observe that the spectral diffusion and the anisotropy both show pronounced biexponential dynamics. On the basis of previous work, the fast component of the spectral dynamics with a time constant of 100 fs is assigned to rapid hydrogen-bond switching events. We observed that these switching events lead to a pronounced effect on the anisotropy of the excited O–D groups, which shows that the spectral relaxation is accompanied by a large change of the orientation of the O–D groups. The slow component of the spectral relaxation can be assigned to the collective structural reorganization of the hydrogen-bond network of liquid water. With increasing temperature, the spectral relaxation shows a similar acceleration as the average molecular reorientation, showing that these processes are intimately connected.