Ion TOF spectra of Ne taken at the photon energy of 93.0 eV and the peak intensity of (a) 4 <b>×</b> 10<sup>15</sup> W cm<sup>−2</sup> and (b) 2 <b>×</b> 10<sup>13</sup> W cm<sup>−2</sup>

<p><strong>Figure 2.</strong> Ion TOF spectra of Ne taken at the photon energy of 93.0 eV and the peak intensity of (a) 4 <b>×</b> 10<sup>15</sup> W cm<sup>−2</sup> and (b) 2 <b>×</b> 10<sup>13</sup> W cm<sup>−2</sup>. In the TOF regime below 2.6 μs, the ion intensities were multiplied by a factor of 30.</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>