In the interaction of intense laser fields with atoms, even subtle details of single ionization and high-harmonic generation can be well described in the so-called single-active-electron approximation where just one electron reacts to the laser field. Therefore, the surprisingly high efficiency of laser-induced nonsequential multiple ionization (where two or more electrons are removed by the field in one coherent process) as opposed to sequential ionization (where they are dislodged one at a time) has raised great interest. Indeed, this premium for cooperation, that is, the ratio of nonsequential over sequential ionization, has been found to be as high as six orders of magnitude in the case of double ionization of helium at near-infrared wavelengths. Ever since these observations were made, the question of the physical mechanism behind this cooperation has been intensely debated, but no final conclusions have been reached.
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