posted on 2014-07-17, 00:00authored bySietse
T. van der Post, Huib J. Bakker
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.