Fig. 1: Sketch map to illustrate the below-threshold nonsequential double ionization process.
From: Recollision of excited electron in below-threshold nonsequential double ionization
At time t1i, e1 is first ionized by the laser field, then it is driven back to collide the parent univalent ion and excites e2 at time t1r. e2 is ionized from the excited state by the laser field at a later time t2i. After that, e2 may travel directly to the detector, or it may be driven back to recollide with the divalent ion similar to e1 at time t2r, or it may also be recaptured into a Rydberg state of ion.
