Third-order S-matrix study of electron-electron correlation in nonsequential double ionization

Correlated electron momentum distribution of nonsequential double ionization of a Ne atom in intense laser fields is calculated using a third-order S-matrix formalism which takes into account the interaction between the Coulomb potential of the parent ion and the photoelectrons. Compared to the second-order S-matrix calculation, the correlation distributions of the two processes which take into account the effect of the ionic Coulomb potential on either of the two ionized electrons after their collision tend to be concentrated off the diagonal of the first and third quadrants, which are closer to the experimental observations. Our further analysis shows that this modification to the second-order calculation is due to the fact that, scattering of the emitted electron upon the parent ion will alter the photoelectron angular distribution from concentration in the laser direction to focus in a direction deviating from the laser field. And this will significantly reduce the momentum of the electron in the direction of the laser field. Our results are in accord with previous studies which reveal that electron-nuclear Coulomb interaction, electron-electron collision and the final-state electron repulsion all play significant roles in the formation of the fingerlike structure. (paper)