Highly doubly excited S states of the helium atom

We calculate and analyse S-wave resonances of helium up to an energy of -0.02 au applying the complex rotation technique. Rydberg series converging to the hydrogenic thresholds of the He⁺ ion are analysed by quantum defect theory. For moderately excited inner electrons the series converging to different thresholds begin to overlap resulting in perturbed Rydberg series. We find that approximate quantum numbers as well as propensity rules governing the decay of the resonances and the perturbation scheme of the Rydberg series prevail. With increasing excitation of the inner electron however, only series whose states tend to extreme interelectronic angles (cos theta )=+or-1 remain regular.