# Compton spectra of atoms at high x-ray intensity

## TL;DR

This paper investigates how high-intensity x-ray pulses from XFELs cause ionization in atoms, significantly altering Compton scattering spectra and impacting single-particle imaging techniques.

## Contribution

It extends the xatom toolkit to include ionization effects, providing new insights into Compton spectra at high x-ray intensities.

## Key findings

- Ionized electrons significantly alter Compton spectra at high fluence.
- Deviations from neutral atom spectra become prominent with increasing x-ray intensity.
- Results inform the interpretation of XFEL-based imaging experiments.

## Abstract

Compton scattering is the nonresonant inelastic scattering of an x-ray photon by an electron and has been used to probe the electron momentum distribution in gas-phase and condensed-matter samples. In the low x-ray intensity regime, Compton scattering from atoms dominantly comes from bound electrons in neutral atoms, neglecting contributions from bound electrons in ions and free (ionized) electrons. In contrast, in the high x-ray intensity regime, the sample experiences severe ionization via x-ray multiphoton multiple ionization dynamics. Thus, it becomes necessary to take into account all the contributions to the Compton scattering signal when atoms are exposed to high-intensity x-ray pulses provided by x-ray free-electron lasers (XFELs). In this paper, we investigate the Compton spectra of atoms at high x-ray intensity, using an extension of the integrated x-ray atomic physics toolkit, \textsc{xatom}. As the x-ray fluence increases, there is a significant contribution from ionized electrons to the Compton spectra, which gives rise to strong deviations from the Compton spectra of neutral atoms. The present study provides not only understanding of the fundamental XFEL--matter interaction but also crucial information for single-particle imaging experiments, where Compton scattering is no longer negligible.