  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, photo-/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.
