Deviations between experimental and theoretical results in Ar (e, 3e) double ionization

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 degrees, 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. (letter to the editor)