ON THE UNIMOLECULAR FRAGMENTATION OF C-60(- THE COMPARISON OF MEASURED AND CALCULATED BREAKDOWN PATTERNS() FULLERENE IONS )

The stability of singly charged C60+ fullerene ions, produced by elect ron impact ionization of C60, has been studied as a function of the el ectron energy and the time elapsed from ionization in a Nier-type ion source/double-focusing, sector-field mass spectrometer system. A huge kinetic shift of more than 34 eV (dependent on the observation time) w as Observed for the dissociation process C60+-->C58++C2. The ionizatio n efficiency curves for C58+, C56+ and C54+ fragment ions have been re corded with an energy resolution of approximately 0.5 eV. This allowed us to construct a time-resolved breakdown graph of the decaying C60fullerene ion. Two different methods, i.e., the finite heat bath model of Klots and the Rice-Ramsperger-Kassel-Marcus (RRKM) expression, hav e been used to calculate the decay rates and the breakdown graph of th e C60+ ion, and the results of the calculation have been compared with the experimentally obtained breakdown graph. The best fit leads to a dissociation energy (C58+-C2) of 7.1+/-0.4 eV. This value is somewhat larger than the dissociation energies predicted earlier from the kinet ic energy release distributions, and possible reasons for this differe nce are discussed in this paper. Using a quasithermochemical cycle the dissociation energy (C58-C2) of the neutral C60 molecule has been est imated to be 7.64+/-0.4 eV.