Resonances Below then=3Doubly Excited States of Helium

Some features have been observed for the first time around 69 eV in the electron-impact differential excitation cross section of the bound state $2{}_{}{}^3{}_{}{}^{}S$ of helium, measured in the forward direction. The main feature, which involves a variation of about 2.8×${10}^{-21\mathrm{}}$ ${\mathrm{cm}}^2$ ${\mathrm{sr}}^{-1\mathrm{}}$ in the cross section, may be assigned to the negative-ion state $3s^{2}3p\left({}_{}{}^2{}_{}{}^{}P\right)$ thus lying at about 68.98±0.07 eV; a second feature is observed at about 69.67±0.08 eV. The detection of these resonances has a special interest because the decay of these ${\mathrm{He}}^{-}$ states involves the three electrons concerned in the collision process.