SMALL CARBON RINGS - DISSOCIATION, ISOMERIZATION, AND A SIMPLE-MODEL BASED ON STRAIN

Injected ion drift tube techniques have been used to examine the prope rties of carbon rings with 10-36 atoms. Previous studies have shown th at the monocyclic ring is the dominant isomer for annealed clusters wi th 10-36 atoms, while for unannealed clusters a bicyclic ring first ap pears at n approximate to 22 and becomes the dominant isomer for n > 3 0. A detailed study of the annealing of the bicyclic ring has been per formed for clusters with 24-36 atoms. Activation energies for the bicy clic to monocyclic transition are around 2.4eV and show a slight syste matic decrease with increasing duster size. The fragmentation of carbo n cluster ions containing 6-30 atoms has been studied and dissociation energies estimated. A simple model for the strain energy of the carbo n rings accounts for many of their physical properties, including isom erization to monocyclic rings, their fragmentation patterns, and disso ciation energies. From simple estimates of their stabilities, planar g raphite fragments should be lower in energy than the monocyclic rings for medium sized clusters (n > 30), but little, if any, of this isomer is observed even after annealing. The low abundance of the graphite f ragments is attributed to the large activation energy (induced by stra in energy) for their formation from monocyclic rings.