Electron-Impact Ionization ofHe(2sS3)

The absolute electron-impact ionization cross section of the metastable $\mathrm{He}\left(2s{}_{}{}^3{}_{}{}^{}S\right)$ state of helium has been measured by the application of a double-electron-beam technique. The cross section at the plateau is about 6.5×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ ± 30%. The ionization of the metastable $\mathrm{He}\left(2s{}_{}{}^1{}_{}{}^{}S\right)$ state of helium has also been observed. This absolute ionization cross-section measurement is in good agreement with the experimental results for lithium and with the theories for $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ when compensation for the differing ionization potentials is made.