Electronic stopping and velocity effect on multiple ionization and fragmentation of C60 in swift heavy ion impacts

Multipleionization and fragmentation of C60 induced by fast heavy ions have been studied by means of triple coincidence measurements of fragment ions, the number of secondary electrons and charge states of outgoing projectile ions. In order to investigate the roles of stopping power Se and of incident velocity v, we used projectile ions of 2.0 MeV Li+, 0.4 MeV O+ and 0.8 MeV Si+, for which Se’s are equivalent (Li+ and O+) and v’s are also equivalent for O+ and Si+ ions. Mass spectra of fragment ions were found to be almost identical in the same v collisions (O+ and Si+) when the number of purely-ionized electrons from C60 is the same. By contrast, in the same Se collisions (Li+ and O+), mass spectra were completely different and the degree of multiple ionization was largely enhanced in O+ impacts. These results indicate evidently that partition of deposited energy Ed between ionization and internal excitation may be strongly dependent on the projectile velocity rather than the stopping power itself. We conclude that, in faster collisions, a larger part of the Ed is spent for electron emission, while in slower collisions Ed may be more converted into internal excitation.