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

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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).