Double ionization of Ar below the recollision threshold intensity

We use the classical ensemble method to analyze the nonsequential double ionization of Ar atoms at a lower laser intensity. The correlated-electron momentum distribution is shown. The electron trajectories from recollision-induced ionization (RII) and from recollision-induced excitation with subsequent ionization (RESI) are studied separately. We find that the momentum of RESI electrons distributes uniformly in the four quadrants of the momentum distribution diagram, while that of RII electrons distributes mainly in the second and fourth quadrants of the momentum distribution diagram. The Coulomb repulsion between the two electrons is responsible for the anticorrelation emission in RII. The momentum distributions of the fast and the slow electrons observed by Liu et al. [Phys. Rev. Lett. 112, 013003 (2014)] are well reproduced in our simulations.