Structural relaxation of hydrogen bonded systems investigated by time-resolved X-ray diffraction

利用超短时间尺度上的时间分辨x射线衍射，可以监测分子液体中的短程有序，提供结构弛豫的信息。对计划中的研究特别感兴趣的是氢键系统的微观动力学，如水、电解水溶液，而简单醇可以作为模型系统进行比较。很明显，水与三维氢键网络相结合的能力导致了这种最重要的溶剂的特殊特征。最近在亚皮秒时间尺度上利用时间分辨红外光谱的实验为更详细地了解水的氢键动力学打开了大门。然而，这些研究的一个重要缺点是，测量的光谱特征必须与结构变化有关，使用迄今为止有争议的经验规则。在亚皮秒时间尺度上，水和相关系统的氢桥键动力学的直接测定是缺乏的。时间分辨小角x射线衍射是一种有效的方法。在过去的几十年里，该技术的时间集成版本已经在许多情况下展示了它的潜力，提供了液体中短时阶的时间和空间平均值。为了获得水的结构动力学，必须开发一种具有亚皮秒分辨率的新型时间分辨版本。将与慕尼黑大学的W. Zinth小组密切合作，利用和建立一种泵-探针方法。在激发过程中，将使用紫外或中红外光谱范围内的飞秒激光脉冲启动样品中的氢键断裂。随后的结构变化由激光等离子体发射的亚皮秒x射线脉冲监测，该激光等离子体由同样的高功率飞秒激光系统产生，该系统也提供泵浦脉冲。新实验将提供与生物系统相关的散装水和含水电解质等氢键系统的超快结构动力学的关键信息。在项目的第二阶段，还将在特殊边界条件下研究水，即在沸石等微孔系统和生物膜模型系统中研究水。

Using time-resolved X-ray diffraction on an ultrashort time scale, the short-range order in molecular liquids can be monitored, providing information on structural relaxation. Of special interest for the planned investigations are the microscopic dynamics of H-bonded systems like water, aqueous electrolytic solutions, while simple alcohols may serve as model systems for comparison. It is clear that the ability of water to associate to a 3-dimensional H-bonded network results in the special features of this most important solvent. Recent experiments utilizing time-resolved infrared spectroscopy on a sub-picosecond time scale have opened the door to a more detailed picture of the hydrogen bond dynamics of water. An important disadvantage in these investigations however is that the measured spectral features have to be related to structural changes using up to now empirical rules which are debatable. A direct determination of the hydrogen bridge bond dynamics of water and related systems on the sub-picosecond time scale is lacking. A powerful method for this purpose is time-resolved small angle X-ray diffraction. The time-integrated version of the technique has demonstrated its potential in numerous cases during the past decades, providing time- and spatial averages of the short-range order in the liquid. To get access to the strutural dynamics of water a novel time-resolved version with sub-picosecond resolution has to be developed. A pump-probe approach will be utilized and built up in close collaboration with the group of W. Zinth at the Munich university. For the excitation process a femtosecond laser pulse in the UV or mid-infrared spectral range will be used initiating the breaking of hydrogen bonds in the sample. The subsequent structural changes are monitored by a sub-picosecond X-ray pulse emitted from a laser plasma generated by the help of the same high power femtosecond laser system that also delivers the pump pulse. The new experiment will provide key information on the ultrafast structural dynamics of H-bonded systems like bulk water and aqueous electrolytes relevant for biological systems. In a second stage of the project, water will be also studied under special boundary conditions, i.e. in microporous systems like zeolithes and in model systems of biological membranes.