High-order S-matrix theory of atomic nonsequential double ionization in intense laser fields

Coulomb corrections to the conventional strong-field approximation (SFA) theory (the lowest-order term of the S-matrix expansion) for nonsequential double ionization are considered by calculating higher-order terms of the S-matrix expansion that take into account multiple Coulomb interactions of the first-ionized electron with the ionic core while the second electron is still in its ground state. For a Ne atom in a linearly polarized laser field, the distribution of the components of the final electron momenta parallel to the field polarization is presented based on the S-matrix expansion up to the sixth order, which involves up to four Coulomb interactions between the parent ion and the first-ionized electron before it collides with the second bound electron. The calculated electron momentum distribution is significantly modified compared with the outcome of the conventional SFA. It is consistent with experimental results and a semiclassical simulation that fully takes into account the effect of the Coulomb potential on the first-tunneled electron. Our analysis shows that multiple Coulomb interactions of the first-ionized electron with the ionic core significantly modify the distribution of the final momenta by favoring larger and nearly equal momenta, in line with experimental results and semiclassical simulations.