Intensity-dependent transitions between different pathways of strong-field double ionization

Momentum distributions of Ne2+ and Ar2+ ions created by linearly polarized 795 nm, 25 fs laser pulses have been traced at intensities from 10(14) to 3x10(15) W/cm(2) using a reaction microscope. Apart from the transition from nonsequential to sequential ionization, characterized by significant changes in longitudinal momentum distributions developing from a double hump, over a triple-peak structure to a narrow single Gaussian observed for both ions, for Ar2+ we find a similar behavior but reversed in its intensity dependence in the purely nonsequential regime, pointing to contributions of recollision excitation plus subsequent field ionization, or to the role of Z trajectories recently predicted within classical calculations.