No abstract

Double ionization of the helium atom by slow electron impact (E(0)=106 eV) is studied in a kinematically complete experiment. Because of a low excess energy E(exc)=27 eV above the double ionization threshold, a strongly correlated three-electron continuum is realized. This is demonstrated by measuring and calculating the fully differential cross sections for equal energy sharing of the final-state electrons. While the electron emission is dominated by a strong Coulomb repulsion, also signatures of more complex dynamics of the full four-body system are identified.

A dynamically screened product of six pairwise Coulomb functions (DS6C) is used as an analytic approximation to describe the four-body Coulomb continuum state produced by electron-impact full fragmentation of helium. Good agreement is obtained with experimental data close to threshold, where four-body effects are expected to be important. Even for the high impact energy of 640 eV, four-body effects still play a role in deciding the shape of multi-differential cross-sections.

We analyse (e, 3e) processes on helium, with one fast and two slow electrons in the final channel, where strong deviations from a first-Born description were reported recently (Lahmam-Bennani et al 2001 J. Phys. B: At. Mol. Opt. Phys. 34 3073–87). The four-particle continuum is described in different approximations leading up to a 6C wavefunction which takes all two-body Coulomb interactions into account. It is shown that deviations from a first-Born description arise from two different effects which are important in two different energy regions.

A simplified derivation of dynamic screening in the case of three Coulomb-interacting particles in the continuum is given and applied to describe the shapes of multi-differential cross-sections for the (e, 3e) process on the helium atom in which the incident fast electron is treated in first-order Born approximation. A very good agreement with experimental multi-differential cross-sections is obtained for two slow ejected electrons (specifically both electrons have 5 eV) but the strategy appears to be breaking down at intermediate emission energies.