Novel supermultiplet energy levels for doubly excited He

The authors propose a novel "supermultiplet" classification of doubly excited states for two electrons in the same shell, constructed from more limited $O(4)$ multiplets of earlier classification schemes. The supermultiplets are found to give a much more efficient classification, and bring to light new features of the spectrum, including accidental near-degeneracies of certain levels having different values of the angular momentum. When viewed with these supermultiplets, the intrashell spectrum is found to bear a striking similarity to a cutoff rotation-vibration spectrum of a linear triatomic molecule. This structure is interpreted with rotations and bending-mode vibrations for a strongly coupled electron pair; energies for these apparent excitations are found to have approximate separability.