Large-scale atomistic calculations of clusters in intense x-ray pulses

Abstract

We present the methodology of our recently developed Monte Carlo/molecular-dynamics method for studying the fundamental ultrafast dynamics induced by high-fluence, high-intensity x-ray free electron laser (XFEL) pulses in clusters. The quantum nature of the initiating ionization process is accounted for by a Monte Carlo method to calculate probabilities of electronic transitions, including photo absorption, inner-shell relaxation, photon scattering, electron collision and recombination dynamics, and thus track the transient electronic configurations (EC) explicitly. The freed electrons, atoms and ions are followed by classical particle trajectories using a molecular dynamics algorithm. Our calculations reveal the role of electron-ion recombination processes that lead to the development of nonuniform spatial charge density profiles in x-ray excited clusters over femtosecond timescales at XFEL intensities exceeding 1020 W cm-2. The Ar cluster fluorescence spectrum is found to be very different from the Ar atom spectrum, in which recombination processes enable additional pathways to reach the required EC for fluorescence transitions. In the high-intensity limit, recombination dynamics can play an important role in the calculated scattering response even for a 2 fs pulse. We demonstrate that our numerical codes and algorithms can make efficient use of the computational power of massively parallel supercomputers to investigate the intense-field dynamics in systems with increasing complexity and size at the ultrafast timescale and in nonlinear x-ray interaction regimes. In particular, picosecond trajectories of XFEL clusters with attosecond time resolution containing millions of particles can be efficiently computed on upwards of 262 144 processes.