Ion TOF spectra of Ne taken at the photon energy of 93.0 eV and the peak intensity of (a) 4 × 1015 W cm−2 and (b) 2 × 1013 W cm−2
Figure 2. Ion TOF spectra of Ne taken at the photon energy of 93.0 eV and the peak intensity of (a) 4 × 1015 W cm−2 and (b) 2 × 1013 W cm−2. In the TOF regime below 2.6 μs, the ion intensities were multiplied by a factor of 30.
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.