Ultrafast memory loss and energy redistribution in the hydrogen bond network of liquid H2O.

Water has many unique features that distinguish it from other liquids. A key to these properties is the dynamic network of hydrogen bonds linking the highly polar water molecules. Infrared spectroscopy of the hydroxyl (OH) stretching vibration is ideal for probing the dynamics and structure of this network, but technical limitations have meant that it has been necessary to use mixtures of isotopically substituted water and observe, for example, the deuteroxyl (OD) bond in normal water (H2O). Now by using an ultrathin sample cell, the behaviour of the hydrogen bond network structure has been studied in pure H2O. The results are surprising: water loses its memory of structural correlations within 50 femtoseconds, an order of magnitude faster than seen previously. This favours rapid relaxation of elementary excitations, which may contribute to the stability of biological systems that strongly interact with surrounding water.