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

点击翻译按钮获取中文摘要

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.