Dissociative and non-dissociative electron capture in medium-velocity cluster-atom collisions; evidence of contributions due to multi-electron processes

Dissociative and non-dissociative electron capture cross sections for medium-velocity collisions of carbon ionic cluster projectiles with helium atoms have been measured (v = 2,2.6 and 4 au Cn+ n≤5). This has been done using a 4π multifragment detector operating either in the full-efficiency mode for the extraction of total electron capture cross sections, or in a partial-efficiency mode (anticoincidence method) for the extraction of the dissociative and non-dissociative parts. The dissociative part has been found to be the largest one in all cases, increasing with n and v. The role of the cluster temperature, not precisely known, may be important but cannot explain all the observed experimental results, in particular the appearance of quite large branching ratios for dissociation into three fragments (n≥3). In order to explain this multifragmentation, the multi-electron process of capture-projectile excitation is invoked. Another multi-electron process is directly extracted from these collisions, namely, the transfer-ionization two-electron process whose relative contribution with respect to single capture is found to increase with the velocity.