Investigation of fast processes in condensed matter by time resolved x-ray diffraction

近年来，利用台式高功率激光产生超短光脉冲来研究激光辐射与物质的非线性相互作用。一个具有挑战性的现象是，在激光强度超过1015W/cm2的情况下，激光能量有效地转化为超热电子的动能(1keV……1 MeV电子能量)。当这些电子穿透冷靶材料时，它们会产生亚皮秒的硬X射线闪光。这种X射线脉冲可用于通过X射线衍射法探测晶体材料的瞬时结构变化。在这里，我们建议利用激光泵浦-X射线探测实验中的特征X射线线在原子尺度上研究晶体的结构重排。这项技术是独特而强大的，因为它允许在几百飞秒的时间尺度上直接观察晶体中的集体原子位移。因此，即使是由快速的电子过程或化学反应引发的原子重排也成为可能。我们建议研究金属和半导体中的快速非热现象，即晶体中声子和光学声子的相干激发。高的时间分辨率允许研究激光激发的电子和声子之间的快速耦合，从而监测激光光子吸收后的快速初始过程。探测X射线脉冲对原始晶格的快速光诱导结构修改将使人们能够从微观上理解这种过程。超导体从正常状态到超导态的转变过程将被研究。此外，可以比以前更精确地测量冲击材料的状态方程。

In recent years, table-top high power lasers producing ultrashort light pulses were used to study the non-linear interaction of laser radiation with matter. One challenging phenomenon is the efficient conversion of laser energy into kinetic energy of supra-thermal electrons (1 keV ... 1 MeV electron energy) at a laser intensity exceeding 1015 W/cm2. As those electrons penetrate the cold target material, they generate a sub-picosecond flash of hard x-rays. Such x-rays pulses may be used to probe transient structural changes of crystalline materials by x-ray diffraction methods. Here, we propose to use characteristic x-ray lines in laser pump - x-ray probe experiments to study structural rearrangement of crystals on an atomic scale. This technique is unique and powerful, since it allows to directly observe collective atomic displacements in a crystal on a time scale of a few hundred femtoseconds. Thus, even the rearrangement of atoms triggered by fast electronic processes or chemical reactions becomes possible. We propose to investigate fast non-thermal phenomena in metals and semiconductors, coherent excititations of acoustic and optical phonons in crystals. The high temporal resolution allows to study fast coupling between laser excited electrons and phonons, thereby monitoring the fast initial processes following the absorption of laser photons. Probing those fast photo-induced structural modifications of the original crystal lattices by x-ray pulses will enable a microscopic understanding of such processes. Such processes will be investigated for superconductors during their transition from normal to superconducting state. In addition, it becomes possible to measure the equation of state of shocked materials with a much higher precision than before.