INHIBITED AUTOIONIZATION OF PLANETARY ATOM STATES

Doubly excited electron states of the helium atom are organized in Rydberg series converging to the various hydrogenic-like thresholds of the He+ ion. As long as the inner electron is not too highly excited these series are well separated in energy. For moderate and high excitation, however, the different Rydberg series overlap energetically and the interference of resonances may lead to a drastic change of the pattern of states emerging at low double excitation. Using very accurate ab initio calculations we show that for asymmetrically highly doubly-excited states (so-called planetary atom states) this interference may lead to surprisingly large lifetimes of the resonances. We relate the almost inhibited autoionization process to propensity rules for decay proposed on the basis of an adiabatic molecular approach.