Probing femtosecond electron dynamics with ultrafast X-ray absorption spectroscopy using a laser-plasma accelerator

Investigating transient phenomena in high-energy-density samples, e.g. the dynamics of electronic structure changes, requires a penetrating ultrafast probe, such as a femtosecond X-ray beam. Here we present ultrafast X-ray absorption spectroscopy (XAS) results from probing the K shell of titanium driven by a high-intensity femtosecond laser.



This was made possible using the betatron X-rays from a laser-plasma wakefield accelerator (LWFA). These sources can generate GeV electron beams and X-ray pulses of >10²³ photons/s/mm²/mrad²/0.1% BW, using many “tabletop” systems. For undertaking ultrafast XAS, these X-rays have the unique combination of a smooth broadband spectrum, but crucially, a pulse duration comparable to the electron bunch, i.e. 10’s femtoseconds.



In a recent experiment, where fast electrons rapidly heated a titanium target to multi-eV temperature, we were able to measure the absorption near the K edge over a spectral range of ~150 eV within a single shot (> 10⁶photons/eV per shot [1]). We observed rapid changes in the absorption spectrum on a sub-picosecond timescale, including a K beta like transition reflecting the ionisation state of the sample, and a modification of the density of states near the edge.



This technique is a powerful tool for probing the instantaneous density of states of HED samples within a single shot. Given that the broadband femtosecond X-ray source can be synchronised with other high-power lasers, this offers the possibilities for studying many ultrafast energetic processes. Future facilities will provide more pump energy in tandem to the LWFA X-ray beam and a wider parameter space of extreme states can be accessed.

[1] Kettle et al. Comms Phys 7, 247 (2024). https://doi.org/10.1038/s42005-024-01735-1