Structural relaxation in supercooled water by time-resolved spectroscopy

Water has many kinetic and thermodynamic properties that exhibit an anomalous dependence on temperature1,2,3,4,5, in particular in the supercooled phase. These anomalies have long been interpreted in terms of underlying structural causes, and their experimental characterization points to the existence of a singularity at a temperature of about 225 K. Further insights into the nature and origin of this singularity might be gained by completely characterizing the structural relaxation in supercooled water6. But until now, such a characterization has only been realized in simulations7,8,9 that agree with the predictions of simple mode-coupling theory10; unambiguous experimental support for this surprising conclusion is, however, not yet available11,12,13,14. Here we report time-resolved optical Kerr effect measurements15 that unambiguously demonstrate that the structural relaxation of liquid and weakly supercooled water follows the behaviour predicted by simple mode-coupling theory. Our findings thus support the interpretation7,8,9 of the singularity as a purely dynamical transition. That is, the anomalous behaviour of weakly supercooled water can be explained using a fully dynamic model and without needing to invoke a thermodynamic origin. In this regard, water behaves like many other, normal molecular liquids that are fragile glass-formers.