Classical collisional trajectories as the source of strong-field double ionization of helium in the knee regime

In this paper, the quasistatic model is extended to describe the double ionization of helium in an intense linearly polarized field, yielding insight into the two-electron correlation effect in the ionization dynamics. Our numerical calculations reproduce the excessive double ionization and the photoelectron spectra observed experimentally both quantitatively and qualitatively. Moreover, it is shown that the classical collisional trajectories are the main source of the double ionization in the knee regime where the double ionization yield is much higher than that predicted by the sequential single active electron models, and responsible for the unusual angular distribution of the photoelectrons.