Classical aspects of laser-induced non-sequential double ionization above and below the threshold.

The S -matrix element for non-sequential double ionization of an atom irradiated by a high-intensity near-infrared laser field via the recollision–impact-ionization scenario is investigated with particular emphasis on a certain classical limit where classical kinematics govern throughout and wave-packet spreading is ignored. The resulting classical model is applied to the calculation of the one-electron energy spectrum in coincidence with double ionization. Various assumptions are made for the interaction by which the returning electron frees the second, up to this time inactive, electron and its initial wave function. The resulting momentum correlations of the two emitted electrons are investigated just above and below the threshold intensity, for which the maximal energy of the recolliding electron equals the ionization energy of the singly charged ion, and the quantum-mechanical amplitude and the classical approximation are compared. The classical model is extended to intensities below this threshold by making allowance for the escape threshold being lowered by the laser field. [ABSTRACT FROM AUTHOR]