Characterization of hydrogen bonded transients in liquid water and ice by ultrafast infrared pump and x-ray absorption probe spectroscopy

普通而独特的液态水可能是地球上最重要、也最不为人所知的物质。连接不断移动的分子的波动的氢键网络决定了水的结构和异常性质。氢键的强度足以连接水分子，但也很弱，在1皮秒（10-12秒）的超快时间尺度上，它们会不断地断裂和重组。了解这些键的快速形成和断裂以及所涉及的分子重排是理解水的结构和性质的先决条件。我们提议用前所未有的实验技术组合来阐明水中氢键的形成和断裂。氢键的断裂将由受控的激光激发引起。氢键网络的后续演变将在分子水平上通过频闪方式（泵-探针方案）通过x射线光谱学作为时间函数进行监测。这将为研究氢键断裂和形成过程中短暂的早期阶段提供独特的途径，并有望显著推进我们对水的理解。

Ordinary yet unique, liquid water is probably the most important and least understood substance on earth. A fluctuating network of hydrogen bonds connecting the constantly mobile molecules determines both structure and anomalous properties of water. The hydrogen bonds are strong enough to link the water molecules but they are also weak enough to constantly break and reform on an ultrafast time scale of 1 picosecond (10-12 seconds). Understanding the fast making and breaking of these bonds and the involved molecular rearrangements is a prerequisite for understanding structure and properties of water. We propose to elucidate the making and breaking of hydrogen bonds in water with an unprecedented combination of experimental techniques. The breaking of hydrogen bonds will be initiated by a controlled laser excitation. The subsequent evolution of the hydrogen bond network will be monitored on a molecular level and as a function of time by x-ray spectroscopy in a stroboscopic fashion (pump-probe scheme). This will give unique access to the short-lived early stages in the processes of hydrogen bond breaking and making and it promises to significantly advance our understanding of water.