Electron correlations in nonsequential double ionization of helium at intensity below the recollision threshold

With a classical three-dimensional ensemble model, we investigate the correlation behavior of the electrons from nonsequential double ionization of helium at intensities below the recollision threshold. At these laser intensities well below the recollision threshold, the two-electron momentum spectra pairs along the laser polarization show anticorrelated behaviors. This anticorrelated behavior becomes stronger when the laser intensity decreases, and then becomes weaker as the laser intensity further decreases. The responsible microscopic dynamics for the anticorrelated behavior and its intensity dependence is explored by back analyzing the double ionization trajectories.