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

这个项目的主要目标是提供激光诱导的超快结构变化的理论描述，并通过超短X射线脉冲对其进行时间分辨分析。特别是，我们建议执行：-模拟硅、锗和InSb的超快相变(固-液和固-固)。-计算在激光诱导的超快结构变化期间，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)谱的时间演化。最近，我们小组发展了一种理论方法，可以模拟任意形状的飞秒激光脉冲在固体中诱导的非平衡、超快相变和烧蚀。该方法已被证明能正确地描述激光诱导的金刚石、石墨和富勒烯的结构变化。我们的方法在于对原子运动的分子动力学模拟，这是基于时间相关的势能面。后者是从微观的电子哈密顿量中得到的，也明确地包括了激光激发和热化效应。我们的目标是首先将这种方法扩展到处理其他材料(Si、Ge和InSb)，并将超快结构变化的分子动力学模拟与可用超短X射线脉冲测量的含时可观测量的计算相结合。我们计划用动力学衍射理论和现代X射线吸收精细结构理论(EXAFS)来计算激光诱导相变和结构变化过程中的摇摆曲线和EXAFS谱的变化。我们的理论项目将与von der Linde/Sokolowski-Tinten(Essen大学)、Sauerbrey(耶拿大学)和Falcon(加州大学伯克利分校)实验小组长期合作制定。

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).