European Lab for Non-Linear Spectroscopy (LENS), Univ. di Firenze, via N. Carrara 1, Sesto Fiorentino, I-50019 Firenze, Italy.
Nat Commun. 2013;4:2401. doi: 10.1038/ncomms3401.
The liquid and supercooled states of water show a series of anomalies whose nature is debated. A key role is attributed to the formation of structural aggregates induced by critical phenomena occurring deep in the supercooled region; the nature of the water anomalies and of the hidden critical processes remains elusive. Here we report a time-resolved optical Kerr effect investigation of the vibrational dynamics and relaxation processes in supercooled bulk water. The experiment measures the water intermolecular vibrations and the structural relaxation process in an extended temperature range, and with unprecedented data quality. A mode-coupling analysis of the experimental data enables to characterize the intermolecular vibrational modes and their interplay with the structural relaxation process. The results bring evidence of the coexistence of two local configurations, which are interpreted as high-density and low-density water forms, with an increasing weight of the latter at low temperatures.
水的液态和过冷态表现出一系列反常现象，其本质仍存在争议。有人认为，过冷区深处发生的临界现象会诱发结构聚集体的形成，在其中扮演着关键角色；而这些反常现象的本质和隐藏的临界过程仍然难以捉摸。在这里，我们报告了一项关于过冷水体振动动力学和弛豫过程的时间分辨光克尔效应研究。该实验在前所未有的高质量数据条件下，测量了过冷水体的分子间振动和结构弛豫过程。通过对实验数据的模式耦合分析，可以对分子间振动模式及其与结构弛豫过程的相互作用进行表征。研究结果表明，两种局部结构（高密度水和低密度水）同时存在，并且随着温度的降低，后者的比例逐渐增加。
