Different sequential photo ionization schemes for neon with 90.5 eV photons (red rectangles): (c) sequential ionization without respecting atomic structure, (b) only energetically allowed ionization steps from the respective ionic ground states, (a) sequential ionization with maximum photon energy storage through shake-up indicated by black rectangles

<p><strong>Figure 4.</strong> Different sequential photo ionization schemes for neon with 90.5 eV photons (red rectangles): (c) sequential ionization without respecting atomic structure, (b) only energetically allowed ionization steps from the respective ionic ground states, (a) sequential ionization with maximum photon energy storage through shake-up indicated by black rectangles. On the left the absolute energy scale of the neon ions (charge state on the right) are given; the ionization potentials IP<sub><em>n</em></sub> in the middle for stepwise ionization from the respective ionic ground state are taken from [<a href="http://iopscience.iop.org/0953-4075/46/16/164025/article#jpb462020bib33" target="_blank">33</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>