Recent measurements of the (e,3-1e) four-fold differential cross sections (4DCS) for double ionisation of helium are here extended to more complex targets, namely neon, argon and molecular nitrogen. The previous observations of large angular shifts in the experimental 4DCS distributions with respect to the momentum transfer axis and the existence of structures in these distributions are found to similarly hold here. For the three investigated targets, the experimental data are compared with the kinematical analysis previously given to describe the second order, 'two-step 2' double ionisation mechanism. Such comparison confirms our interpretation which allows relating the observed shifts and structures in the intensity distributions mostly to the 'two-step 2' mechanism, which is shown to predominate over the first-order 'shake-off' and 'two-step 1' mechanisms under the present kinematics. The experimental data are also compared to the predictions of a first Born and a second Born model, showing a rather mixed agreement.

Double ionization of nitrogen molecules is investigated in coplanar asymmetric (e,3-1e) and (e,3e) experiments. The measurements are performed at intermediate incident energy (∼600 eV) with one fast scattered electron and two slower ejected electrons sharing equally the excess energy. The data are compared with the predictions of a simple kinematical model recently reported by Lahmam-Bennani et al (2010 J. Phys. B: At. Mol. Opt. Phys. 43 105201) which describes the 'two-step 2' (TS2) mechanism as two successive (e,2e) single ionization interactions. The model is extended to include binary as well as recoil scattering during the two successive single ionization events. It is shown to qualitatively predict the correct angular positions for most of the observed structures. Moreover, the (e,3e) data are compared with the predictions of a first Born model which fails to reproduce the experimental angular distributions. These results demonstrate that the molecular double ionization process is largely dominated by the TS2 mechanism, as was the case in our previous works on atomic targets.

New measurements of the electron impact (e,3-1e) fourfold differential cross section for double ionization of isoelectronic atomic and molecular targets, namely Ne and CH 4 , are reported. The measurements are performed at intermediate incident energy and a different energy range for the pair of ejected electrons. We compare the experimental results with the predictions of a recently developed kinematical model (Lahmam-Bennani et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 105201), which can explain the appearance of the structures related to the second-order mechanism (such as TS2). Some of the data are compared with the predictions of the first and second Born models, showing mixed agreement. Moreover, the targets are isoelectronic hence the difference between the behaviour of the cross sections is qualitatively discussed in terms of the molecular structure influence over the ionization process.