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

Abstract

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 C + n 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 partialefficiency 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.

Fragmentation branching ratios of highly excited hydrocarbon molecules CnH and their cations CnH+ (n⩽4)

“…Cross sections of electronic processes measured in the collision of C n H + projectiles with helium are reported in figure 2 as a function of n. The ionization process always dominates, followed by dissociative electronic excitation, and electron capture. The relative importance of these processes is as expected in this velocity range 34,35 .…”

“…This last form, depending on four parameters a 1 -a 4 , is known to reproduce the energy distribution due to electron capture in any velocity range 29 . We note that the internal energy distribution is very broad, extending from low energies under the dissociation limit up to large energies, well above the ionization potential of C 4 H. We obtained a similar result in the case of carbon clusters and explained this broad distribution by the electron capture process itself which, in high velocity ion-atom collisions, tends to populate an extended range of excited states 24,34 .…”

“…The same conclusion was reached on smaller systems studied with photons 52 . In C 4 , fragmentation branching ratios measured in high velocity collisions 34 are close to those obtained in dissociative recombination 53 . As to the fragmentation of cationic carbon clusters C n + (n=4-10), similar results were found for measured branching ratios in photodissociation and in high velocity collisions 54 .…”

Structure, Dissociation Energies, and Harmonic Frequencies of Small Doubly Charged Carbon Clusters Cn2+ (n = 3−9)

“…The most basic question to be answered in this field is the following: what is the minimum size a carbon cluster must have in order to be stable against Coulomb explosion for a given value of charge q ? Wohrer and collaborators − have recently shed some light into this problem. They have produced highly ionized carbon clusters, C n q + with q ≥ 2, in collisions of fast singly ionized carbon clusters impinging on a helium gas.…”

Electron capture in the ion-cluster collision: effect of the electronic interaction

“…Another recent experiments [11] have been reported on the dissociative and non dissociative electron capture cross section, but for medium velocity collision of carbon cluster C + n (n < 5) with helium. For the range of velocity the dissociative part has been found to be the largest one in all cases increasing with n. In order to explain the quite large branching ratios for dissociation, it is very interesting to notice that the multielectron process of capture-projectile excitation has been invoked [11].…”