Double ionization of helium in fast ion collisions: the role of momentum transfer

Double ionization of helium in the perturbative regime has been explored in a kinematically complete collision experiment using 100 MeV/u C6+ ions. Di fferent ionization mechanisms are identified by inspecting the angular dist ribution of the electrons as a function of the momentum transfer q to the t arget by the projectile. For q < 1.2 au, both electrons are distributed uni formly in the plane perpendicular to the projectile axis, and distinct simi larities with photoionization are identified. For q > 1.2 an, the faster el ectron resulting from a binary encounter with the projectile is emitted alo ng the direction of momentum transfer, while the other electron is distribu ted uniformly. Experimental data are compared with various model calculatio ns based on the Bethe-Born approximation with shake-off. Surprisingly, the effect of the final state interaction is found to depend decisively on the choice of the initial state wavefunction.