Mass spectrometric studies of fullerene ion beams

The energetics and dynamics of C-2 evaporations from carbon cluster ions C-n(+) were studied using time-resolved mass spectrometric techniques. Time-resolved appearance energies (AEs) were determined for C-58(+) and C-56(+) on a double-focusing mass spectrometer using electron-space-charge trapping. The AEs were found to be nearly independent of time for times longer than 80 mu s. A large intrinsic kinetic shift, IS=30.5 eV, was observed. This intrinsic shift is due to radiative decay in the visible, which is unusually efficient for fullerenes. Time-resolved kinetic energy releases (KERs) and metastable fractions (MFs) for C-n(+) (n = 48 -62) were measured on an ion trap/reflectron mass spectrometer for trapping times up to 100 mu s. The average KER for C-2 evaporation from fullerenes was observed to decrease with increasing precursor ion lifetime. This constitutes proof that the KER is due to the non-fixed energy in the transition state rather than to a reverse activation energy, and the reaction takes place via a loose transition state. The metastable fractions decrease with increasing trapping time, as expected. The results obtained for C-58(+) --> C-56(+) + C-2 are successfully modeled taking into account radiative cooling. The shape of the plot of MFs versus In(t(2)/t(1)) (where t(1) and t(2) are the times corresponding to the beginning and the end of a metastable time window, respectively) is mainly defined by the relation between dissociative and radiative rate constants.