Search for positron quasibound states in the doubly excited region of the helium atom

Abstract

Recent calculations by Bromley have indicated that the He(2s21Se) state will bind a positron. The e+He(2s21Se) state has a binding energy of 0.447 eV with respect to the He(2s21Se) state. Binding of a positron to the He 2s2 state has been predicted to give rise to two resonances in positron scattering from ground-state helium: e+He(2s21Se) at 57.3716 eV and a 2Po shape resonance just above the He(2s21Se) threshold at 57.8485 eV. To investigate these predicted resonances, low-energy positron scattering experiments were performed to measure the total and positronium cross sections in the doubly excited region of helium in this energy region. No experimental evidence has been found for these resonances, but upper bounds on the resonance strength were determined for both predicted resonances.