(a) The PAD of the MCP signals for Ar<sup>4 +</sup> detection and the fitting result

<p><strong>Figure 3.</strong> (a) The PAD of the MCP signals for Ar<sup>4 +</sup> detection and the fitting result. On the left, the graph is zoomed in to 0 showing the number of events for 'detecting no ion'. (b) Comparison between the pulse counting, the Poisson distribution constructed by 'detecting no ion' and the fitting for the PAD of the MCP read-out signals for Ar<sup>4 +</sup> detection.</p> <p><strong>Abstract</strong></p> <p>We have investigated multiphoton multiple ionization of argon and xenon atoms at 5 keV using a new x-ray free electron laser (XFEL) facility, the SPring-8 Angstrom Compact free electron LAser (SACLA) in Japan. The experimental results are compared with the new theoretical results presented here. The absolute fluence of the XFEL pulse has been determined with the help of the calculations utilizing two-photon processes in the argon atom. The high charge states up to +22 observed for Xe in comparison with the calculations point to the occurrence of sequential <em>L</em>-shell multiphoton absorption and of resonance-enabled x-ray multiple ionization.</p>