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

普通而又独特的液态水可能是地球上最重要也是最不为人所知的物质。不断移动的分子之间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，但它们也足够弱，可以在1皮秒（10-12秒）的超快时间尺度上不断断裂和改革。理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。我们建议用前所未有的实验技术来阐明水分子中氢键的形成和断裂。氢键的断裂将由受控的激光激发引发。随后的氢键网络的演变将在分子水平上进行监测，并作为时间的函数，通过X射线光谱在频闪的方式（泵探测方案）。这将为氢键断裂和形成过程中短暂的早期阶段提供独特的途径，并有望显着提高我们对水的理解。

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.