Observation of low-lying resonance states of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">−</mml:mi></mml:mrow></mml:msup></mml:mrow></mml:math>at the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>S and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>S He thresholds

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.