Near threshold electron-impact double ionization of helium

Abstract

We examine the break-up pattern for the electron-impact double ionization of helium using a time-dependent close-coupling approach. This ionization process is one of the simplest examples of the Coulomb four-body problem of three charged particles moving in the Coulomb field of a bare nucleus. Studies of this four-body process in the near-threshold region allow exploration of the break-up dynamics, which have been stimulated by differing predictions of the most likely geometries for break-up in the related processes of triple photoionization of lithium and electron-impact double ionization of helium. While both processes lead to the same final state of three electrons moving in the field of a bare nucleus, the preferred break-up pattern at low energies for the triple photoionization is predicted as a T-shape (Emmanouilidou et al 2008 Phys. Rev. Lett. 100 063002; Colgan et al 2013 Phys. Rev. Lett. 110 063001) and for the electron-impact double ionization as a ‘triangle’ pattern (Ren et al 2008 Phys. Rev. Lett. 101 093201). Our calculations at an excess energy of 5 eV show that a triangle break-up pattern is favored, confirming earlier experimental results (Ren et al 2008 Phys. Rev. Lett. 101 093201).