Time-Dependent Multiphoton Ionization of Xenon in the Soft-X-Ray Regime

The time-dependent multiphoton ionization of xenon atoms is studied with femtosecond pulses in the excitation range of the $4d$ giant resonance at the photon energy of 93 eV. Benefiting from a new operation mode of the free electron laser FLASH, the measurements are performed with varying pulse durations. A strong dependence of the ion charge distribution on the pulse duration allows the different multiphoton mechanisms behind the multiple photoionization of xenon to be disentangled up to a charge state of Xe¹⁰⁺. The results up to Xe⁸⁺ are well explained by sequences of single photon, multiphoton, and Auger processes, but higher charge state generation suggests the need for collective electron multiphoton excitations.