  In this paper, a quasistatic model is extended to describe the double
ionization of Helium in intense linearly polarized field, yielding achieve an
insight to 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
and responsible for the unusual angular distribution of the photoelectrons.
