Observation of low-lying resonance states ofHe−at the21S and23S He thresholds

Abstract

We measured the cusp electron production associated with target ionization at the impact of a 400-keV pure2${\mathrm{}}^{3}$S He beam and a mixed beam containing all three long-lived He states, i.e., 1${\mathrm{}}^{1}$S,n2${\mathrm{}}^{1}$S, and 2${\mathrm{}}^{3}$S. Using the results of an earlier experiment [Kuzel et al., Phys. Rev. A 48, R1745 (1993)], we estimated the cross section for both metastable states of He. We found that the cusp for the 2${\mathrm{}}^{1}$S state is much larger and sharper than for the 2${\mathrm{}}^{3}$S state. The peaks are manifestations of excitation of low-lying virtual or weakly bound states of the ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ ion at the 2${\mathrm{}}^{1}$S and 2${\mathrm{}}^{3}$S thresholds.