Elucidating the mechanisms of double ionization using intense half-cycle, single-cycle, and double half-cycle pulses

We investigate the interaction of a two-active electron system $\left({\mathrm{Li}}^{-}\right)$ with intense single-cycle and double half-cycle pulses. The “intensity” and “frequency” considered correspond to the “multiphoton above-barrier regime.” For the single-cycle pulse (SCP), the electric field changes sign once, allowing electron wave packets created during the first half cycle to recollide with the parent ion when driven back by the field. For the double half-cycle pulse (DHP), however, the electric field does not change sign, and electron wave packets created during the first half cycle are not driven back to the parent ion. We find that both single and double ionization are significantly larger for the SCP than for the DHP, thereby elucidating the role of the rescattering mechanism. On the other hand, doubly ionized electrons produced by a half-cycle pulse and a DHP are found to have angular distributions in which one electron is ejected in the direction of the pulse field, and the other in the opposite direction. This clear signature of electron correlations suggests that “shake-off,” “knockout,” and, possibly, “multiphoton-sharing” processes are alternative contributing mechanisms for double ionization in this regime.