The role of multi-return induced collision-ionization in atomic nonsequential double ionization

The nonsequential double ionization of helium by high-intensity (1.5×1015 W/cm2) few-cycle laser pulses is investigated using a three-dimensional semi-classical rescattering model, with emphasis on the longitudinal correlated momentum spectra. We find that the percentage of opposite-hemisphere emission in this case is remarkably higher than those produced by laser pulses with medium-intensity (e.g., 7×1014 W/cm2) and low-intensity (e.g., 2.5×1014 W/cm2) those procuced by the laser pulses pulses with, and also the same intensies but longer laser pulses. Meanwhile, the V-shape structure is more pronounced. We identify the underlying mechanisms with tracing back the individual trajectories, and find that the single-and the multi-return induced collision-ionization are mainly responsible for the opposite- and the same-hemisphere emission, respectively. The dependences of these two different mechanisms on peak intensity and pulse length are predicted.