Excitation of Atomic Hydrogen to the Metastable $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ State by Electron Impact

The cross section for excitation of atomic hydrogen to the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state by electron impact has been remeasured in a modulated-crossed-beam experiment in the energy range extending from threshold to 1000 eV. Absolute values for the $2S$ cross section were obtained by measuring the ratio of the $2S$ to $2P$ cross sections and using previous measurements of the $2P$ cross section, which were normalized to the Born approximation at high energies. The atoms excited to the $2P$ and $2S$ states were detected by observing the Lyman-$\alpha$ photons produced by radiative decay and by quenching in an electrostatic field, respectively. In order to determine the total cross section, it was necessary to consider the polarization of the Lyman-$\alpha$ radiation emitted in each method of detection. The remeasured cross section is found to be in agreement with the Born approximation above 200 eV. The measured ratios are in reasonable agreement with the ratios that would be predicted by the close-coupling theory of Burke et al. The peak value of the cross-section curve is $0.168\pm 0.020\pi {a_0}^2$, and occurs at 11.6±0.2 eV.