Helium atom in a box:: Doubly excited levels within the S-wave model -: art. no. 022703

A complete nonperturbative close-coupling solution of the helium atom in a box problem is presented by developing two numerical techniques. The first technique is the direct solution by diagonalization of the Hamiltonian, and the second is based on a constrained relaxation of the wave functions. A Feshbach projection operator of the direct solutions to the bound-continuum subspaces allows a comparison of the low-lying autoionization levels obtained in both methods. Time-dependent propagation of these doubly excited wave functions is analyzed, allowing the calculation and the visualization of the autoionization process. In this work, results are presented for the S-wave model in which the electrons are restricted to spherical states and all angular correlations have been eliminated.