The relative cross sections of Ar double ionization by electron impact have been measured in an (e, 3e) experiment at low incident energy E0 = 561.4 eV, at a scattering angle θa = 6.5° and with equal energy sharing among the two `ejected' electrons, Eb = Ec = 9 eV. The experimental results are compared with calculations in the first Born approximation, which include the shake-off and two-step 1 mechanisms in the target-projectile interaction. The symmetry breaking about the direction of the momentum transfer and the appearance of a small feature near the main ones show that the non-first-order effects (such as two-step 2 mechanism) play an important role. These effects are enhanced in the present experiments with respect to the former ones (El Marji et al 1997 J. Phys. B: At. Mol. Opt. Phys. 30 3677) with higher collision energy, E0 = 5563 eV, which is in agreement with an earlier theoretical estimate by McGuire.

New coplanar (e, 3e) experiments for double ionization (DI) of helium (1s2) and argon (3p2) at about 600 eV incident energy, and under moderate momentum transfer to the target (~0.8 au), are presented. It is shown that, when the data are sorted according to the Bethe-ridge condition (i.e. the magnitude of the momentum transfer is equal to that of the sum momentum of the emitted electrons), the (e, 3e) angular distributions clearly bear the signature of the target initial two-electron momentum density, |ψ(p1,p2)|2. We show, for the first time in DI, the different signature due to the s or p character of the ionized electrons. For He, the results are qualitatively compared with a state-of-the-art first-order theory.

The coplanar (e, 3e) relative cross sections for double ionization of argon have been measured at an electron impact energy of E0 = 561.4 eV and under equal energy sharing among the two ‘ejected’ electrons, Eb = Ec = 9 eV. The scattering angle is fixed to θa = 1.5°, corresponding to a momentum transfer K = 0.4 au to the target. The experimental results have been compared with calculations in the first Born approximation, which include only first-order processes in the target–projectile interaction. The comparison shows severe deviations between the experimental and theoretical results. These deviations are much larger than the ones previously observed in helium under comparable kinematics. To fill this gap between theory and experiment, a decisive improvement in the theory is needed. This can be achieved by improving the first-order calculations and by including higher-order processes such as the two-step mechanism, or even new mechanisms, for instance the simultaneous ejection of the pair of target electrons.

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