Laser ablation of fullerene films results in coalescence of fullerenes to form large carbon clusters. Mobility studies of size-selected C-120(+) formed by this method reveal the presence of at least two abundant structural isomers. One of these isomers has a closed-shell fullerene structure (identical to that generated by the laser vaporization of graphite), and the other has a C-60 "dimer" structure. When these clusters are collisionally annealed, around 40% of the "dimer" dissociates to give C-60(+) + C-60 and around 30% appears to anneal into the closed-shell fullerene. The balance remains unchanged up to relatively high injection energies, indicating the formation of some strongly bound "dimers". The activation energies for the annealing and dissociation processes are low, approximately 1.5 eV.