Abstract
The authors present a theoretical description of the multiple ionisation of atoms. The description is based on the Thomas-Fermi model. They show that if the ionisation process is non-resonant and if the laser frequency omega L is smaller than 1015 s-1, the time-dependent external electric field can be replaced by the static one. Ionisation is due to the lowering of the potential barrier by the external field. The authors show that multiply charged ions occur as a result of a stepwise process. Multiple ionisation appears after the saturation of the singly charged ions. The saturation intensities of the electric field obtained in the model are in very good agreement with the experimental data. Contrary to the existing models, the model gives a very simple physical picture of the ionisation and has no free parameters fitted to the experiment.

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