Time-dependent diffraction and EXAFS during laser induced ultrashort structural changes: theory and simulations

该项目的主要目标是通过超短X射线脉冲提供激光诱导超快结构变化及其时间分辨分析的理论描述。特别是，我们建议执行：-在硅，锗和InSb的超快相变（固-液和固-固）的模拟。- 计算激光诱导超快结构变化过程中随时间变化的X射线衍射特性和随时间变化的X射线吸收精细结构（EXAFS）谱。最近，我们的小组已经开发出一种理论方法，允许模拟非平衡，超快相变和烧蚀引起的固体飞秒激光脉冲的任意形状。该方法已被证明是正确的描述激光诱导的金刚石，石墨和富勒烯的结构变化。我们的方法包括原子运动的分子动力学模拟，这是基于随时间变化的势能面。后者是从微观电子哈密顿量，也明确包括激光激发和热化效应。我们的目标是首先扩展这种方法来处理其他材料（硅，锗和InSb），并结合联合收割机的分子动力学模拟的超快结构变化与时间相关的可观测量的计算，可以使用超短X射线脉冲测量。我们计划利用动力学衍射理论和现代X射线吸收精细结构理论（EXAFS）计算激光诱导相变和结构变化过程中摇摆曲线和EXAFS谱的变化。我们的理论项目将与实验小组von der Linde/Sokolowski-Tinten（埃森大学），Sauerbrey（耶拿大学）和Falcone（加州大学伯克利分校）永久合作。

The main goal of this project is to provide a theoretical description of laser induced ultrafast structural changes and of their time-resolved analysis through ultrashort x-ray pulses. In particular, we propose to perform: - Simulations of ultrafast phase transitions (solid-liquid and solid-solid) in Silicon, Germanium and InSb. - Calculation of the time evolution, during laser induced ultrafast structural changes, of time-dependent x-ray diffraction properties, and time-dependent x-ray absorption fine structure (EXAFS) spectra. Recently, our group has developed a theoretical approach which permits to simulate nonequilibrium, ultrafast phase transitions and ablation induced in solids by femtosecond laser pulses of arbitrary shape. The method has been shown to describe correctly laser induced structural changes in diamond, graphite and fullerenes. Our approach consist in molecular dynamics simulations for the atomic motion, which are based on time dependent potential energy surfaces. The latter are obtained from a microscopic electronic Hamiltonian and also include explicitly the laser excitation and thermalization effects. Our goal is first to extend this method to treat other materials (Si, Ge and InSb), and to combine the molecular dynamics simulations of the ultrafast structural changes with the calculation of time dependent observable quantities which can be measured using ultrashort x-ray pulses. We plan to use dynamical diffraction theory and modern theory of x-ray absorption fine structure (EXAFS) to calculate the changes of rocking curves and EXAFS-spectra during laser induced phase transitions and structural changes. Our theoretical project will be worked out in permanent collaboration with the experimental groups von der Linde/Sokolowski-Tinten (Universität Essen), Sauerbrey (Universität Jena) and Falcone (University of California, Berkeley).