Effect of the electron initial longitudinal velocity on the nonsequential double-ionization process

The effect of initial longitudinal velocity of the first tunneling electron on the nonsequential double-ionization (NSDI) process is studied using a semiclassical model. Our analysis shows that nonzero initial longitudinal velocity affects two distinct typical trajectories [single-return collision (SRC) and multiple-return collision (MRC)] in the NSDI process in different ways: for SRC, the yield in the center part of the correlation momentum distribution is reduced; for MRC, the tunneling time of the first electron is apparently delayed. In comparison with the case of zero initial longitudinal velocity, the double ionization is suppressed for a long wavelength, and a better agreement with the experimentally observed electron correlation distribution is achieved when the nonzero initial longitudinal velocity is taken into account.