Multiple ionization of krypton at around L edges by X-ray free-electron laser

The extremely high peak power of an X-ray free electron laser pulse can be an attractive tool for clarifying the core-level excitation and relaxation process. Recently, Dr. B. Rudek and his colleagues have reported their time-of-flight ion spectroscopy studies on sequential inner-shell multiple ionization of krypton at photon energies at 2 keV and 1.5 keV, which are higher than the LI (~1.92 keV) and lower than the LIII (~1.67 keV) edges for ordinary neutral krypton, respectively. The experiments were done with two X-ray pulse widths (5 and 80 fs) and various pulse energies (from 0.07 to 2.6 mJ), at the Linac Coherent Light Source (LCLS), Stanford, USA. The highest charge state observed at 1.5 keV photon energy (below the LI edge) is Kr¹⁷⁺; at 2 keV photon energy (above the LIII edge), it is Kr²¹⁺. It was found that theoretical calculations based on a rate-equation model can explain the obtained experimental data for 1.5 keV, but fails to do so at 2 keV, where the experimental spectrum shows higher charge states. They discussed that this enhancement is due to a resonance-enhanced X-ray multiple ionization mechanism, i.e., resonant excitations followed by autoionization at charge states higher than Kr¹²⁺, where direct L-shell photoionization at 2 keV is energetically closed. For more information, see the paper, "Resonance-enhanced multiple ionization of krypton at an x-ray free-electron laser", B. I. Cho et al., Phys. Rev. A87, 023413 (2013).