Distinction of direct and sequential multiple ionization of neon by intense soft XUV femtosecond pulses

Abstract

In the situation where the slopes of atomic ionization yields as a function of the laser intensity do not permit to distinguish between direct and sequential multiple photoionization processes, an analysis of the magnitude of those ionization yields enable such distinction. Using an analytical rate equation model we show that comparing these magnitudes by introducing reduced ionization yields leads to clustering curves in the sequential case and spreading in the direct one when laser focus averaging is taken into account. Applying this insight to the experimental data, we are able to assess that the sequential process is responsible for ionization of neon up to Ne⁵⁺ induced by short, intense and coherent soft XUV light provided by free-electron laser sources.