Annealing and dissociation of carbon rings

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Abstract
Laser vaporization of graphite generates carbon clusters which possess a number of different geometric structures ranging from planar ring systems to fullerenes. We have employed a multicollision excitation scheme combined with injected ion drift tube techniques to examine the dissociation and annealing of carbon cluster ions. In this paper, we focus on clusters containing 30–50 atoms. Nonfullerene cluster ions in this size range can be annealed in the gas phase to just two dominant isomers—the fullerene and an isomer which appears to be a large monocyclic ring. Fragmentation by loss of C2 units is associated with annealing into the fullerene. This process occurs at low injection energies and rapidly becomes more important with increasing cluster size. However, relatively few clusters follow this path in the size range examined here (less than 3% for C40+). Most of the nonfullerene isomers anneal into a large monocyclic ring which (for clusters in the size range examined here) does not appear to convert into the fullerene. At higher injection energies, a second fragmentation route is observed. This second dissociation process seems to be associated with cleavage of the large monocyclic rings into two roughly equal fragments. Activation energies for these processes are estimated, and the possible role they play in the gas phase synthesis of fullerenes is discussed.