Changes in a Silicon Crystal Under Intense X-rays

What is it about?

We study how silicon responds to extreme conditions caused by intense X-ray pulses from Free Electron Lasers. A previous experiment showed an unexpected decrease in the scattering signal as the x-ray intensity increased. Using simulation tools, we reproduce these results and identify the mechanisms responsible for the signal reduction. Our focus is on electron dynamics, meaning how electrons get excited and removed from atoms during interaction with the pulse. Electron impact initiates an ionization cascade that rapidly removes outer-shell electrons from silicon atoms. Once enough electrons are removed, impact excitation promotes the remaining bound electrons to move into the outer shell, from which they can be efficiently ejected. These processes happen within the duration of the pulse, which lasts only a few quadrillions of a second (1 femtosecond, or 1E-15 seconds), and collectively contribute to the observed decrease in scattering signal. By examining a range of intensities, we identify a threshold at which the scattering signal reduction begins. We also find that thermalization of electrons lags and influences these results.

Featured Image

Photo by Tsuyoshi Kozu on Unsplash

Photo by Tsuyoshi Kozu on Unsplash

Why is it important?

This investigation is relevant to continue to develop imaging techniques such as Serial Femtosecond X-ray Crystallography, which enables atomic-resolution structural analysis of biomolecules using micro- and nano-sized crystals. By benchmarking simulation models against experimental data, we can uncover gaps in our understanding of radiation damage mechanisms.