Fullerene collisions and clusters of fullerenes

Electron capture processes were studied in glancing collisions between multiply charged C60q+ ions and neutral C60 molecules at low collision energies where nuclear stopping dominates the interaction (typical energy: 10 keV; velocity v=0.024 a.u.v=0.024 a.u.). In addition, clusters of fullerenes were multiply ionized in collisions with highly charged slow ions and their fragmentation spectra were measured by applying multi-coincidence techniques in a second, separate experiment. Through the first experiment we show that non-fragmenting electron capture collisions do not produce free electrons in C60q+–C60 collisions (q = 1–5), i.e., the cross sections for transfer ionization processes are negligible in this charge state regime. This is in contrast to the case of atomic projectile ions where transfer ionization processes, as e.g., Cq+ + C60 → C(q−r)+ + C60r+ → C(q−r+1)+ + C60r+ + e−, are strong for q > 2. These results are rationalized by means of the classical over-the-barrier model for electron transfer between two C60 molecules, or between an atomic ion and a C60 molecule, where the molecules are modeled as conducting spheres. Further, the same model may also be used as a basis for understanding the present observations of limitations in the maximum numbers of charges, which might be transferred in non-fragmenting C60q+–C60 collisions (q/2 for even q and (q + 1)/2 for odd q) and in Cq+–C60 collision (here up to q charges may be transferred). In the same experiment, we have further measured scattering angles, θ, and energy losses, ɛ, for the fullerene projectiles in C60q+–C60 collisions and we have found low values for both θ and ɛ, which, however, increase with the number of C2-units lost from the projectile fullerene in electron capture collisions. The critical distances for electron transfer which are deduced from the C60q+–C60 collision experiment and the Zettergren model are then used to explain the high charge mobility between the individual C60 molecules in charged (C60)n van der Waal's clusters of fullerenes, which we observe in the second experiment.