Impact of ionization potential depression on single particle imaging

Ultraintense and ultrashort x-ray free-electron laser (XFEL) pulses promise single particle imaging (SPI) via enabling collection of diffraction data for nanocrystals, or nano-size nonperiodic objects before the destruction of the sample. Photoionization and subsequent processes lead to plasma generation within the sample in such experiments. The continuum energy levels of electrons for atoms and atomic ions are lowered due to plasma screening, also known as ionization potential depression (IPD). We theoretically investigate the plasma formation and the effect of IPD in the context of SPI with calculations performed on bulk glycine, mimicking the interior of irradiated biological macromolecules, e.g., proteins or viruses. To simulate the plasma formation dynamics, we employ a nonequilibrium, hybrid quantum-classical approach, combined with the evaluation of the transient IPD from first-principles electronic structure calculations considering the time-dependent microscopic environment, which in earlier work was applied to a solid-density plasma consisting of a single atomic species []. Here, this approach is extended to more than one atomic species for applications to biological macromolecules in SPI studies. Our work quantifies the effect and importance of IPD in XFEL-based imaging of biological systems and provides further guidance for simulations of electronic radiation damage dynamics toward successful SPI experiments.