Title: Interplay between relativistic energy corrections and resonant excitations in x-ray multiphoton ionization dynamics of Xe atoms,
Abstract: In this paper, we theoretically study x-ray multiphoton ionization dynamics
of heavy atoms taking into account relativistic and resonance effects. When an
atom is exposed to an intense x-ray pulse generated by an x-ray free-electron
laser (XFEL), it is ionized to a highly charged ion via a sequence of
single-photon ionization and accompanying relaxation processes, and its final
charge state is limited by the last ionic state that can be ionized by a
single-photon ionization. If x-ray multiphoton ionization involves deep
inner-shell electrons in heavy atoms, energy shifts by relativistic effects
play an important role in ionization dynamics, as pointed out in [Phys.\ Rev.\
Lett.\ \textbf{110}, 173005 (2013)]. On the other hand, if the x-ray beam has a
broad energy bandwidth, the high-intensity x-ray pulse can drive resonant
photo-excitations for a broad range of ionic states and ionize even beyond the
direct one-photon ionization limit, as first proposed in [Nature\ Photon.\
\textbf{6}, 858 (2012)]. To investigate both relativistic and resonance
effects, we extend the \textsc{xatom} toolkit to incorporate relativistic
energy corrections and resonant excitations in x-ray multiphoton ionization
dynamics calculations. Charge-state distributions are calculated for Xe atoms
interacting with intense XFEL pulses at a photon energy of 1.5~keV and 5.5~keV,
respectively. For both photon energies, we demonstrate that the role of
resonant excitations in ionization dynamics is altered due to significant
shifts of orbital energy levels by relativistic effects. Therefore it is
necessary to take into account both effects to accurately simulate multiphoton
multiple ionization dynamics at high x-ray intensity.