vapor was bombarded by and ions in the energy range 420–625 MeV to study the various ionization and fragmentation processes that occur. Since the center-of-mass energies used in this work exceeded those of previous studies by several orders of magnitude, new excitation and dissociation modes were expected and indeed found. Positive ions were extracted from the interaction region and their times of flight were measured both singly and in coincidence with other ionic fragments. A wide range of stable charge states and cluster sizes from monatomic carbon up to was observed. Even-numbered carbon fragments dominated the heavier mass range but both even and odd carbon numbers occurred at lower masses. Evidence was found for three qualitatively different ionization and fragmentation channels suggesting different ranges of collision impact parameters: ionization of the parent molecule, loss of even numbers of carbon atoms, and ‘‘multifragmentation’’ into many small fragments. This latter mode included the production of singly charged fragments with all values of n being observed from n=1 up to at least n=19. We interpret our results in terms of a theoretical model that indicates that the total interaction cross section contains comparable contributions from (a) excitation of the giant dipole plasmon resonance, and (b) large-energy-transfer processes that lead to multiple fragmentation of the molecule. The distribution of fragment cluster masses for n≲20 is reproduced by a ‘‘percolation theory’’ description analogous to that used to describe multifragmentation of nuclei by high-energy protons. © 1996 The American Physical Society.
- Received 11 March 1996
- Published in the issue dated October 1996
© 1996 The American Physical Society