Structures in the Low-Energye−-He Scattering Cross Sections

A cylindrical, retarding potential difference type of electron spectrometer with high-energy resoltuion has been used to study the structures in the $e^{-}$-He scattering cross section in a transmission experiment. Twenty-four structures have been observed between the transmission maximum due to ${\mathrm{He}}^{-}$ $1s2s^2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ at 19.30±0.01 eV and ${\mathrm{He}}^{+}$ $1s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ at 24.60±0.02 eV, 11 of which have been observed previously. The agreement with previous measurements for the positions of the structures is good where comparison is possible. Four of the new structures have been observed below the $n=2\mathrm{}$ states of He. The excitation onsets for $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$, $2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$, $2{}_{}{}^3{}_{}{}^{}P_{}^0{}_{}{}^{}$, and $2{}_{}{}^1{}_{}{}^{}P_{}^0{}_{}{}^{}$ have been observed, as well as some excitation onsets at higher energies. No structures have been observed below ${\mathrm{He}}^{-}$ ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$. Two structures have been observed above ${\mathrm{He}}^{+}$ ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ which were observed previously.