Electron impact excitation of the3S,3P, and3Dstates of H

Abstract

The relative excitation function for electron-impact excitation of H to yield Balmer $\alpha$ has been measured from threshold to 500-eV collision energy in a crossed-beam apparatus. The relative contributions of the $3S$,$3P$, and $3D$ states to this excitation function have been measured by modulating the electron beam and discriminating on the basis of their different lifetimes for radiative decay after excitation. The resulting set of relative $3S$,$3P$, and $3D$ cross sections has been normalized by equating the total measured Balmer-$\alpha$ cross section at 500 eV to the Born approximation. The normalized cross sections are then compared to available theories at lower energies. In contrast to virtually all known excitation cross sections of neutral atoms, which fall below the Born theory at low energies, the H ($1S\to 3D$) cross section exceeds the Born theory, apparently owing to coupling between the $3D$ and $2P$ states.