A new type of He- resonance states in the autoionisation region

The authors have studied the near-threshold excitation of autoionising states in electron-helium scattering. Only a particular set of autoionising states is observed in the measurements, which has led them to formulate a selection rule for threshold excitation of autoionising states. Evidence is presented that autoionising states belonging to this particular set are strongly excited near their thresholds by hitherto unknown shape resonances