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 mediu m-velocity collisions of carbon ionic cluster projectiles with helium atoms have been measured (nu = 2, 2.6 and 4 au C+nz less than or equal to 5). Th is has been done using a 4 pi multifragment detector operating either in th e full-efficiency mode for the extraction of total electron capture cross s ections, or in a partial-efficiency mode (anticoincidence method) for the e xtraction of the dissociative and non-dissociative parts. The dissociative part has been found to be the largest one in all cases, increasing with n a nd nu. The role of the cluster temperature, not precisely known, may be imp ortant but cannot explain all the observed experimental results, in particu lar the appearance of quite large branching ratios for dissociation into th ree fragments (n greater than or equal to 3). In order to explain this mult ifragmentation, the multi-electron process of capture-projectile excitation is invoked. Another multi-electron process is directly extracted from thes e collisions, namely, the transfer-ionization two-electron process whose re lative contribution with respect to single capture is found to increase wit h the velocity.