Modeling electron dynamics in silicon driven by high-intensity femtosecond x-rays

Area of Science:

• Materials Science
• Atomic Physics
• X-ray Crystallography

Background:

• High-intensity, femtosecond X-rays from free electron lasers enable novel crystallography with smaller crystals.
• Increasing X-ray pulse intensity causes ultra-fast changes in atomic scattering factors due to electron dynamics.
• Previous theoretical studies of silicon diffraction at high X-ray intensities have not been fully reproduced.

Purpose of the Study:

• To computationally investigate the ionization dynamics and diffraction efficiency of silicon under high-intensity X-ray free electron laser (XFEL) pulses.
• To understand the underlying mechanisms responsible for changes in diffraction efficiency observed in experiments.
• To bridge the gap between theoretical models and experimental results for intense X-ray interactions.

Main Methods:

• Utilized collisional radiative simulations coupled with relativistic configuration-averaged atomic data.
• Incorporated ionization potential depression by adjusting energy levels near the ionization threshold.
• Examined the influence of free electron degeneracy and non-thermal electron distributions.

Main Results:

• Achieved good agreement between simulations and experimental results for silicon diffraction efficiency within experimental uncertainty.
• Identified key electron impact ionization mechanisms driving the observed phenomena.
• Found that electronic damage affects diffraction efficiency at both high and low momentum transfer regimes.
• Determined that free electron degeneracy does not significantly influence ionization dynamics.

Conclusions:

• The study provides a robust computational model for understanding X-ray-induced ionization and diffraction in silicon.
• The findings clarify the mechanisms behind diffraction efficiency changes at high X-ray intensities.
• This research offers valuable insights for optimizing future experiments using intense XFEL pulses for high-resolution structural determination.