Resonance Structure in the Excitation Cross Section by Electron Impact of the2sState in Atomic Hydrogen

The cross section for excitation of atomic hydrogen to the $2s$ state by electron impact has been remeasured in a crossed-beam experiment in the energy range from threshold to 12.3 eV. An electron monochromator of the cylindrical type with a full width at half-maximum of 110 meV has been used. The $2s$ atoms were detected by quenching in an electrostatic field and observing the emitted Lyman-$\alpha$ photons. Absolute values for the cross section were obtained by normalizing to the close-coupling calculations of Burke et al. at 10.8 eV. Resonance structure at 11.7 eV, which is a superposition of the ${}_{}{}^1{}_{}{}^{}S$ and ${}_{}{}^3{}_{}{}^{}P$ resonances predicted by theory, has been detected. There is good agreement between the present experiment and the experiment by Oed.