Experimental data for Ne ionization yields measured at photon energies of 93.0 (▪) and 90.5 eV () with pulses of 15 fs duration compared with results from the present minimal model ({full}) and results from the elaborate rate description for 30 fs pulse length of [20] (- − − −)

Figure 3. Experimental data for Ne ionization yields measured at photon energies of 93.0 (▪) and 90.5 eV () with pulses of 15 fs duration compared with results from the present minimal model ({\full}) and results from the elaborate rate description for 30 fs pulse length of [20] (- − − −).

Abstract

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.