Excitation to the metastable states and ionization from ground and metastable states in helium : measurements with crossed atomic and electron beams

Abstract

We designed a crossed beam apparatus, the new aspect of which is that an atomic beam is subsequently crossed by two parallel electron beams. The first electron beam has the function to produce metastable atoms which are then ionized in the second beam. In this way we measured the energy dependence of the cross sections for ionization of He from the metastable 23S and 21S states. These measurements require a high ion detection efficiency.

The high sensitivity of the apparatus also enabled us (i) to measure cumulative ionization within one electron beam (and to measure excitation to the 23S and 21S states of He) and (ii) to very accurately measure the cross section for direct ionization from the ground state. We reduced the effect of thermal energy spread in the electron beam on the latter experimental results using mathematical unfolding techniques. For direct ionization from the ground state of He we obtained the following threshold dependence of the cross section Qi (in 10-16 cm2) on the electron energy Ee (in eV):

Qi = (8.6 ± 0.5) × 10-3 (Ee - 24.58) 1.13 ± 0.02 for 24.58 eV ≤ Ee ≤ 27.1 ± 0.3 eV, and

Qi = 0.0125 [Ee - (25.14 0.04)] for 27.1 ± 0.3 eV ≤ Ee ≤ 34 eV. These formulae agree with experiment within the experimental uncertainty. We cannot decide whether a more complicated formula would give a better fit.