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.

Figure 1

The calculated momentum distributions of the fast and the slow electrons for the side-by-side emission (a) and the back-to-back emission (b), respectively. Black solid line in (a) denotes the data after smoothing.

Figure 2

The momentum distributions of electrons (a) the drift momentum and (b) the initial momentum, respectively. Black solid lines denote the data after smoothing. The momentum distribution of the second electron is almost the same as that of the first electron. The change of the kinetic energy with time for (c) the RII electrons and (d) the RESI electrons, respectively. The red and blue lines denote different electrons.

Figure 3

Correlated longitudinal momentum distributions for the two electrons in NSDI (a) and in RESI only (b). The dash-dot line represents $y=-x$. Interior squares denote $\pm 2\sqrt{U_p}$.

Figure 4

The variation of the energy (a), (e), the transverse momentum (b), (f), the longitudinal momentum (c), (g) and the coordination in polarization direction (d), (h) with time, respectively. Left column is for the RII electron and right column is for the RESI electron, respectively. The black dashed curves mark the electric field in arbitrary units.