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The electron capture process in high velocity cluster-atom collisions revisited

Members of the “Astrophysics and molecular assemblies” group at ISMO have studied, in collaboration with researchers at IPNO, the ability of high velocity neutral projectiles (atoms, clusters) to capture electrons from atoms. The case of MeV Cn projectiles colliding with helium atoms was investigated using the AGAT set-up. The authors showed that, in these collisions, cross sections leading to anionic Cn-species are quite large, only 5 to 10 times smaller than corresponding electron capture cross sections by incident cationic Cn+ depending on n. Apart from its fundamental interest (no measurements at all existed with neutral polyatomic projectiles) the authors could show that account of this process allowed to interpret previous unexplained results. This work has been published recently in J.Phys.B:At.Mol.Opt.Phys. 46 (2013) and highlighted by the journal.

The experimental set-up is based on a coincident recording of all fragments issued from the collision, identified by their mass and charge (negative, neutral or positive). In order to separate double electron capture by Cn+projectiles in one collision from two successive single electron capture processes (by Cn+ in a first collision and by Cn in a second collision), the authors performed a He target thickness dependence study. With the extracted single electron capture cross sections in C, C+ - He collisions they revisited the neutralization process in Cn+ - He collisions, treating Cn+ as (n-1) C atoms and a C+ ion (Independent Atom and Electron model). What they found is that the experimental increase of neutralization cross sections (see figure below) so far unexplained (Chabot et al J.Phys.B (2006)) has to be entirely attributable to electron capture by neutral carbon atoms (difference between the solid and broken lines), indeed a capital process !
sections efficace de neutralisation de projectiles
In the same paper, the authors reported on double electron capture process from He by Cn+ projectiles. They measured the intact fraction of produced Cn- anions and all dissociative branching ratios of the fragmented part. The C2- fragment was found prominently populated (magical fragment).