  Consensus has been reached that recollision, as the most important
post-tunneling process, is responsible for nonsequential double ionization
process in intense infrared laser field, however, its effect has been
restricted to interaction between the first ionized electron and the residual
univalent ion so far. Here we identify the key role of recollision between the
second ionized electron and the divalent ion in the below-threshold
nonsequential double ionization process by introducing a Coulomb-corrected
quantum-trajectories method, which enables us to well reproduce the
experimentally observed cross-shaped and anti-correlated patterns in correlated
two-electron momentum distributions, and also the transition between these two
patterns. Being significantly enhanced relatively by the recapture process,
recolliding trajectories of the second electron excited by the first- or
third-return recolliding trajectories of the first electron produce the
cross-shaped or anti-correlated distributions, respectively. And the transition
is induced by the increasing contribution of the third return with increasing
pulse duration. Our work provides new insight into atomic ionization dynamics
and paves the new way to imaging of ultrafast dynamics of atoms and molecules
in intense laser field.
