Completely understanding of the dynamics of three-body Coulomb system, i.e., non-sequential double ionization, is a challenging task and represents one of incompletely solved problems of theoretical and experimental physics, holding a variety of puzzles on both classical and quantum levels. It is of paradigmatic importance since it reveals complicated and fundamental electron dynamics in atomic and molecular physics. Especially, the liberation of two or more electrons from an atom by strong laser fields is dominated by the electron correlation. Extensive studies of atoms and molecules in strong infrared light fields have revealed that electron correlation dominates the enhancement of double ionization yields at moderate laser intensity. Much effort has been concentrated on exploring physical mechanism of strong-field non-sequential double ionization.

Recently, Prof. Yunquan Liu and Prof. Qihuang Gong experimentally collaborated with Prof. Ullich at Max-Planck-Institute for Nuclear Physics, and have performed a triple coincidence study on differential momentum distributions of strong-field double ionization of Ar atoms in linearly polarized fields (795 nm, 45 fs, W/cm2). Using a three-dimensional two-electron atomic-ensemble semiclassical model including the tunneling effect for both electrons, we retrieve differential momentum distributions and achieve a good agreement with the measurement. Ionization dynamics of the correlated electrons for the side-by-side and back-to-back emission is analyzed separately. According to semiclasscial model, we find that the doubly excited states are largely populated after the laser-assisted recollision and large amounts of double ionization dominantly takes place through sequential ionization of doubly excited states at such low laser intensity. This work has provided the complete characterization of strong-field double ionization and was published on Phys. Rev. Lett. 112,013003(2014) (http://prl.aps.org/abstract/PRL/v112/i1/e013003).

We acknowledge the supports from the National Program on Key Basic Research Project. Yunquan acknowledges the support by the National Science Fund for Distinguished Young Scholars in China.