Single-photon cross sections σ<sub><em>jk</em></sub> in units of 10<sup>−18</sup> cm<sup>2</sup>, from experiment [26, 30] (bold), using the GIPPER codeprotecthyperlink{jpb462020fn1}{$^{6}$} [31] (italic) and a fitting procedure [32]

<p><b>Table 1.</b> Single-photon cross sections σ<sub><em>jk</em></sub> in units of 10<sup>−18</sup> cm<sup>2</sup>, from experiment [<a href="http://iopscience.iop.org/0953-4075/46/16/164025/article#jpb462020bib26" target="_blank">26</a>, <a href="http://iopscience.iop.org/0953-4075/46/16/164025/article#jpb462020bib30" target="_blank">30</a>] (bold), using the GIPPER code\protect\hyperlink{jpb462020fn1}{$^{6}$} [<a href="http://iopscience.iop.org/0953-4075/46/16/164025/article#jpb462020bib31" target="_blank">31</a>] (italic) and a fitting procedure [<a href="http://iopscience.iop.org/0953-4075/46/16/164025/article#jpb462020bib32" target="_blank">32</a>].</p> <p><strong>Abstract</strong></p> <p>At the free-electron laser FLASH, multiple ionization of neon atoms was quantitatively investigated at photon energies of 93.0 and 90.5 eV. For ion charge states up to 6+, we compare the respective absolute photoionization yields with results from a minimal model and an elaborate description including standard sequential and direct photoionization channels. Both approaches are based on rate equations and take into account a Gaussian spatial intensity distribution of the laser beam. From the comparison we conclude that photoionization up to a charge of 5+ can be described by the minimal model which we interpret as sequential photoionization assisted by electron shake-up processes. For higher charges, the experimental ionization yields systematically exceed the elaborate rate-based prediction.</p>