Electron impact ionisation of metastable atomic hydrogen

Abstract

The authors report on measurements of the electron impact ionisation cross section of metastable atomic hydrogen in the energy range 6.3-998.3 eV. Crossed electron and atom beams are used in a new method where the more usual intensity modulation is replaced by a 'see-saw' motion of the electron beam across the atom beam. The main advantage of the method is that it obviates the problem of the density distribution in the beams. Absolute results, with 10% accuracy, are obtained for the difference ( sigma 2s- sigma 1s) between the cross sections of the atom in the metastable and ground states. At high energies, the results are consistent with the Bethe formula, in contrast with earlier experimental data. At low energies large discrepancies subsist with all theoretical predictions.