OBSERVATION OF LOW-LYING RESONANCE STATES OF HE- AT THE 2-S-1 AND 2-S-3 HE THRESHOLDS

We measured the cusp electron production associated with target ioniza tion at the impact of a 400-keV pure 2, S-3 He beam and a mixed beam c ontaining all three long-lived He states, i.e., 1 S-1, 2 S-1, and 2 S- 3. Using the results of an earlier experiment [Kuzel et al., Phys. Rev . A 48, R1745 (1993)], we estimated the cross section for both metasta ble states of He. We found that the cusp for the 2 S-1 state is much l arger and sharper than for the 2 3S state. The peaks are manifestation s of excitation of low-lying virtual or weakly bound states of the He- ion at the 2 S-1 and 2 S-3 thresholds.