Vector correlations in dissociative charge transfer induced in He+-O2, N2collisions at medium energies

The dissociative ionization of O 2 and N 2 molecules following electron capture in He + -O 2 , N 2 collisions has been investigated in the keV energy range with a new device which combines imaging and time-of-flight resolved coincidence techniques. The ( V He + , V He , V O + (N) + ) velocity vector correlation consists of measuring the V He and V O + (N) + velocities of the He scattered neutral and O + (N + ) fragment ion produced by the same collision event, for a well defined V He + incoming projectile velocity. It leads to a kinematically complete analysis of the collision. The subsequent kinetic energy correlation provides direct identification of all the quasi-resonant, exoergic and endoergic processes in terms of their reaction pathway, namely the intermediate molecular ion excited state in the Franck-Condon region and the corresponding dissociation limit. The spatial analysis of the velocity vector correlation for the dissociative ionization events enables us to investigate for each process the dependence of the electron capture probabilities upon the alignment of the molecular axis with respect to the V He + collision velocity and to the collision plane. Significant spatial anisotropies are observed for the endoergic process involving the excitation of the N 2 + ( F 2 Σ g + ) state in He + -N 2 collisions at 1.5 and 3 keV, and visualized in the (χ c , ϕ c ) angular correlation diagram, where χ c and ϕ c are the polar and azimuthal angles which characterize the molecular axis orientation in the collision frame.