Maximum correlation at the transition to the saturation regime of nonsequential double ionization

The nonseqential double ionization of helium in the laser with intensities ranging from blow the recolliding ionization threshold to near the sequential double ionization regime is studied systematically. We obtain the probability ratio between correlation and anti-correlation of the two final electrons, and find the maximum ratio is in the transition region to the saturation regime of nonsequential double ionization. The maximum ratio generation course is shown in correlated two-electron longitudinal momentum spectrum, and the deep mechanism is investigated by orbit back-trace analysis. The rescattering of one electron after recollision and the high speed recollision of the tunneled electron lead to the increases of anticorrelation ratio in low- and high- intensity laser fields respectively.