Mechanism of Resonance-enhanced X-ray Multiple Ionization of Argon Atom in an XFEL Pulse

We present a new Monte Carlo rate equation (MCRE) approach to examine the inner-shell ionization dynamics of atoms in an intense x-ray free-electron laser (XFEL) pulse.~ In addition to photoionization, Auger decay and fluorescence processes, we include bound-to-bound transitions in the rate equation calculations.~ This computational tool allows us to account for ``hidden resonances" [1] unveiled in high charge states of atom in XFEL pulse.~ Using our MCRE approach, we investigated the ionization dynamics of Argon atom exposed to an 480-eV XFEL pulse.~ At this photon energy, it is not energetically allowed to produce Ar ions with charge 10$+$ and higher via direct one-photon L-shell ionization.~ Rather, we found that the resonance-enhanced x-ray multiple ionization (REXMI) pathways [2] play a dominant role in producing these highly charged ions.~ Our calculated results agree with the measured Ar ion yield data [3].~ More importantly, we account for the pulse-duration dependence of experimental ion yield data and identify the responsible REXMI pathways where excitation of multiple electrons into outer valence and Rydberg orbitals are followed by autoionization. [1] E. Kanter \textit{et al}. Phys. Rev. Lett \textbf{107}, 233001 (2012). [2] B. Rudek \textit{et al}. Nat. Photonics \textbf{6}, 858 (2012). [3] S. Schorb \textit{et al}. Phys. Rev. Lett \textbf{108}, 233401 (2012).