Dynamics of He double ionization in the non-perturbative regime: the reduction to an effective three-particle problem

Double ionization of He by 3.6 MeV u^-1 Au⁵³⁺ 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 He²⁺ 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 ionization 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 ionization mechanism are drawn.