Atomic and molecular autoionizing states. A theoretical approach

We present a progress report on a new method to calculate positions and widths of two-electron atomic (molecular) resonances. This procedure, based on a formal analogy between the Feshbach formalism and the Phillips-Kleinman pseudopotential approach, yields results in very good agreement with those obtained with the standard Feshbach formalism, but requires a considerably smaller computational effort in cases where the resonant states of interest lie above several ionization thresholds. This method can be very easily generalized to the treatment of doubly excited states of Be-like systems, that is containing a closed shell 1s2 core. In our implementation of the method, the open channel wavefunction is obtained from using a discretization procedure, whose theoretical basis we present in some detail, and an inverse interpolation method to achieve the discrete-continuum degeneracy which is needed to apply the golden rule formula to calculate the width. We present some illustrations for He-like and Be-like resonances lying above one or several ionization thresholds, and for molecular resonances of homo and heteronuclear two-electron systems