CARBON CLUSTER ANIONS - STRUCTURE AND GROWTH FROM C-5(-) TO C-62(-)

Carbon cluster anions are generated by laser desorption in a Smalley-type cluster source over a wide size range. The ions are mass selected and their isomer distributions and structures determined as a function of size, from C-5(-) to C-62(-), using ion chromatography. The families of structures found were identical to those previously found for cation clusters, although the ranges of stability are quite different. For example, linear C-n(-), ions are detected to n = 30, while they are not detected above n = 10 for cations. The quantitative growth sequence is the same as for cations; linear at small sizes, transforming to a series of planar ring systems at intermediate values of n, followed by a very dow emergence of fullerenes at the largest values of n sampled. For n near 60, fullerenes comprise less than 20% of the isomers and planar rings more than 80%. For cations, fullerenes comprise more than 95% of the isomer distribution for even n in this size range. A number of possible explanations for this behavior are put forward. Annealing studies are performed for 10 less than or equal to n less than or equal to 30. At the low end of this size range annealing leads to pure linear isomeric distributions, but above n = 15 monocyclic rings begin to dominate. Between n = 20 and 30 annealing yields complete conversion of bicyclic planar rings to monocyclic rings.