Dynamics of He Double Ionisation in the Non Perturbative Regime: The Reduction to an Effective Three-Particle Problem

Double ionisation of He by 3.6 MeV/u Au impact is investigated in a kinematically complete experiment using an integrated multi-electron recoil-ion momentum spectrometer (reaction microscope). Surprisingly, the final-state correlation between the recoiling He target ion momentum and the momenta of both emitted electrons is found to be the strongest among the various two-body correlations. On this basis it is demonstrated that the four-body momentum balance can be reduced to a good approximation to an effective three-particle problem by considering the centre-of-mass motion of the two electrons instead of their individual momenta. Then, all essential dynamical features observed for single ionisation earlier, like for example a strong forward-backward asymmetry in the longitudinal momentum balance resulting from the final-state interaction with the projectile, are naturally rediscovered. Moreover, important conclusions on the properties of the TS-2 double ionisation mechanism are drawn. PACS numbers: 34.10+x, 34.50.Fa