Monte Carlo calculation of ion, electron, and photon spectra of xenon atoms in x-ray free-electron laser pulses

When atoms and molecules are irradiated by an x-ray free-electron laser (XFEL), they are highly ionized via a sequence of one-photon ionization and relaxation processes. To describe the ionization dynamics during XFEL pulses, a rate equation model has been employed. Even though this model is straightforward for the case of light atoms, it generates a huge number of coupled rate equations for heavy atoms like xenon, which are not trivial to solve directly. Here, we employ the Monte Carlo method to address this problem and we investigate ionization dynamics of xenon atoms induced by XFEL pulses at a photon energy of 4500 eV. Charge-state distributions, photoelectron and Auger electron spectra, and fluorescence spectra are presented for x-ray fluences of up to ${10}^{13}$ photons/$\mu$m${}^2$. With the photon energy of 4500 eV, xenon atoms can be ionized up to $+$44 through multiphoton absorption characterized by sequential one-photon single-electron interactions.

Figure 1

Orbital binding energies of the ground configuration of Xe and its charge states. The symbols (circle, triangle, and square) indicate that the corresponding subshell contains at least one electron.

Figure 2

Pathways of 100 exemplary trajectories of ionization dynamics of Xe at 4500 eV, 80 fs FWHM, and 5$\times$10${}^{12}$ photons/$\mu$m${}^2$. Multiphoton multiple ionization is described by a sequence of one-photon ionization (blue or dark bar), Auger decay (green or light bar), and fluorescence (yellow dot). The gray background shows the Gaussian pulse profile of 80 fs FWHM.

Figure 3

Charge-state distribution of Xe at 4500 eV as a function of the fluence.

Figure 4

Electron spectra of Xe at 4500 eV as a function of the fluence. The photoelectrons are above 1250 eV and the Auger (Coster-Kronig) electrons are below 1250 eV.

Figure 5

Fluorescence spectra of Xe at 4500 eV as a function of the fluence. The peak assignments are explained in the text.

Figure 6

Selected fluorescence energies of Xe as a function of the charge state.