Transient grating experiment on supercooled water

We measured the acoustic and thermal relaxation dynamics on supercooled water in the temperature range from 42 to -17degreesC by heterodyne-detected transient grating (TG) experiments. The TG signal shows some particular and interesting features that are produced by the anomalous behaviour of the thermodynamic parameters of water. In particular, the thermal diffusion effects disappear in the TG data for a temperature of about 0degreesC. We found that this effect has to be ascribed to the combination of two different physical phenomena: the anomalous temperature dependence of water density and the contribution to the TG signal from the dielectric variation induced directly by the temperature. This term is usually neglected in both light scattering and TG studies; our data show clearly a non-negligible thermal decay that has to be addressed. The data are analysed using a model based on a simple formulation of the linearized hydrodynamic equations. A Markovian approximation for the memory function is used, since the structural relaxation times are much faster than the other responses analysed.