XFEL fluence dependence of the ion yields for Ar^{<em>n</em> +} (<em>n</em> = 3, 4, 8 and 9)

<p><strong>Figure 5.</strong> XFEL fluence dependence of the ion yields for Ar^{<em>n</em> +} (<em>n</em> = 3, 4, 8 and 9). Closed circles with error bars depict the experimental results and solid lines depict the theoretical results. Lines with slope <em>p</em> = 1 and 2 are also shown as broken lines to guide the eye.</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.