Uncovering Mechanisms Behind Electronic and Structural Changes Under High-Intensity Femtosecond X-ray Exposure
Year: 2025
Type: dissertation
Abstract: We can use x-rays to study the electronic and atomic structure and dynamics of materials because they are surface-sensitive, offer elemental contrast, and their wavelength in the hard x-ray regime approaches the length of atomic bonds. The development of x-ray free-electron laser sources capable of producing x-ray pulses with up to terawatt peak power and femtosecond durations opens up an opportunity to explore new scientific phenomena. The large number of photons in a single pulse rearranges the electronic structure of atoms, deposits large quantities of energy, and drives the material into a nonequilibrium state within femtosecond timescales. This thesis theoretically examines the interactions of dense matter with high-intensity ultrafast x-rays to identify the primary mechanisms responsible for various experimental observations, using collisional-radiative simulations based on rate equations, as well as far-field diffraction simulations. In one set of projects, we investigate the nonlinear effects manifested in changes to copper's absorption, experimentally measured via transmission of the incident pulse and x-ray emission spectra. Severe ionization increases the atom's effective charge and brings the remaining bound electrons radially closer to the core. This leads to shifts in the K-shell absorption edge and the emergence of a transient K-shell to M-shell resonance. In the second set of projects, we examine the x-ray emission spectra from plasma that emerges under extreme x-ray exposure. To understand its formation and evolution over time, we use detailed atomic data and demonstrate the role that electron impact has on the emission spectra from highly charged neon several picoseconds after the pulse. As a proof of concept, we also train a neural network to link how the x-ray pulse duration and fluence affect the x-ray emission spectra from lysozyme nanocrystals. In the last set of studies, we examined the electronic and structural rearrangement of matter relevant for coherent diffractive imaging experiments. We model electronic changes that decrease the diffraction efficiency in silicon crystals within a few femtoseconds at high intensities, as well as the atomic shifts that lead to the anisotropic nonthermal melting of hexagonal ice. Through simulations, we replicate and elucidate the mechanisms that are essential for gaining a richer understanding of the various electronic and atomic dynamics triggered by short x-ray pulses, which can guide future experiments.
Author: Cardoch, Sebastian
Institution: Uppsala University
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7079898224