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The regime of highly doubly excited states of a two-electron atom might be considered as one of the most important unsolved fundamental problems in atomic physics. Doubly excited states of two-electron atoms cannot in general be described by simple models based for instance on independent-particle quantum numbers. On the contrary, a comprehensive understanding of the general structure of the energy spectrum and eigenstates in the energy region at and immediately below the double ionization threshold requires, from a theoretical point of view, sophisticated approaches. Such an approach for planar helium [1] is used here for the description of the spectrum of highly doubly excited states. Photoionization cross sections calculated up to the 20th single ionization threshold exhibit fluctuations. These are mainly due to a dominant series of resonances which can be associated with an approximate quantum number F=N-K in accordance with 3D full calculations and experimental observations [2]. As the energy increases the dominant role of a single series as sole contributor is apparently lost as new series start to contribute significantly to the cross sections. |