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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 a = 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.

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