Intense x-rays provide extraordinary spatial resolution and elemental specificity to enable tools that can resolve the dynamics of atoms and electrons. X-ray free-electron laser (XFEL) pulses, such as those recently available at the upgraded Linac Coherent Light Source (LCLS-II), Spring-8 Angstrom Compact Free Electron Laser (SACLA) facilities, the European-XFEL, Swiss-FEL and Korean PAL-XFEL provide unprecedented intensity pulses with femtosecond and shorter duration. These pulses can potentially enable the generation of real-time “movies” that can follow, in three dimensions, key ultrafast processes during chemical reactions in liquids, advanced materials, and biological systems. As new multimodal experimental techniques are being developed rapidly, concurrent theoretical development, as proposed here, will enable significant advances in imaging and the study of dynamical processes of materials.

For 3D flash imaging with atomic resolution and elemental contrast, it is critical to fully understand the fundamental interactions of these intense x-ray pulses with nanosized heterogeneous systems (e.g., inner-shell ionization, relaxation, charge transfer and electron-ion recombination). A thorough understanding of XFEL pulse-mediated electronic transitions and the ensuing response of the environment serves not only to realize the LCLS Single Particle Imaging Initiative, but also guide the design of new, unique experiments and light source facilities. The planned research work is an important step forward in understanding high-brightness, high energy, coherent x-ray laser pulses and their interactions with materials, supporting the Department of Energy’s (DOE) mission to extend the scientific and technological strengths of the U.S.

This project will examine multimodal atomic imaging approaches enabled by the most intense femtosecond and attosecond XFEL pulses. In particular, this project will investigate and control resonant scattering processes for gain in signals and imaging resolution with intense femtosecond and attosecond x-ray pulses. To complement the scattering approach, the planned simulation campaigns will explore the potential of x-ray correlation methods by investigating fluorescence intensity correlations, based on the principle of Hanbury Brown and Twiss effect, for imaging structure and elemental contrast in heterogeneous systems.

The challenges associated with tracking the motion of particles and evolution of electronic configurations are addressed with a novel Monte-Carlo/Molecular-Dynamics simulation algorithm implemented in the highly parallelized simulation code LAMMPS as required for the large multimillion particle systems under experimental study. The quantum nature of the initiating ionization process is accounted for by a Monte Carlo method to calculate probabilities of electronic transitions and thus track the transient electronic configurations explicitly. The freed electrons and ions are followed by classical particle trajectories using a molecular dynamics algorithm. The combination of this novel simulation method with leadership computational resources will facilitate the efficient investigation of the truly multiscale nature of these complicated processes in heterogeneous systems. The chosen xenon clusters and doped iron oxide nanoparticle systems are directly connected with the planned experimental efforts at current light facilities. The results from these simulations will provide predictions and new concepts to guide multimodal measurements using XFEL, and at the same time, maximize use of limited LCLS-II resources.