Regular Article

Absolute total cross sections for electron-impact double ionization of He(1s2s ³S) and He⁻(1s2s2p ⁴P)^★

Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, Louvain-la-Neuve 1348, Belgium

^a e-mail: matthieu.genevriez@phys.chem.ethz.ch

Received: 16 October 2018
Received in final form: 11 December 2018
Published online: 7 February 2019

Abstract

We report on the experimental determination of absolute, total cross sections for electron-impact double ionization of helium in its metastable 1s2s ³S state and of the helium negative ion He⁻(1s2s2p ⁴P). Cross sections are measured for impact energies ranging from threshold to 1000 eV using the animated crossed beams technique. The fast, pure beam of metastable helium required for the experiment is produced by photodetachment of fast He⁻ ions. Results for He(1s2s ³S) are compared to those for He(1s^{2 1}S), and exhibit similar magnitude and behaviour. The cross section for He⁻ is large compared to other negative ions and indicates the predominance of indirect ionization channels at large impact energies.