Role of high ponderomotive energy in laser-induced nonsequential double ionization

The laser-induced nonsequential double ionization (NSDI) of rare gas atoms in the near and mid-IR laser fields is studied experimentally and theoretically. We investigate electron-electron correlation at high recollision energies, experimentally achieving ponderomotive energies (Up) above 80 eV. The contribution of the two dominant channels of NSDI in the photoelectron momentum distribution, impact, and excitation ionization, are both shown to scale with ponderomotive energy and are well reproduced by theory. Surprisingly, for a large Up in mid-IR fields, a noticeable electron-electron anticorrelation signal emerges at low photoelectron momenta, which cannot be explained by these mechanisms within state-of-the-art theoretical approaches.