Kinetic Energy Releases and Electron-Induced Decay of C 60 z+

Using a high performance electron gun, electron-induced ionization and excitation of fullerene ion beams is achieved in the second field-free region of a reversed-geometry double-focusing mass spectrometer. Excitation of mass-selected C₆₀^z+ ion beams leads to consecutive C₂ eliminations, which are observed in mass-analyzed ion kinetic energy (MIKE) spectra. Electron-induced decay of the ions takes place on a short time scale of 0–3 μs. Average kinetic energy release (KER) values were determined for the electron-induced C₂ eliminations from C₆₀^z+ ions with z = 2–4 and were invariably found to be higher than for the corresponding spontaneous eliminations. The results are interpreted as being due to a time-dependent KER, since the non-fixed energy in the transition state is increased for higher internal energies and shorter lifetimes of the reactant ions. These results are compared with time-resolved KER values obtained previously, using an ion trap/reflectron instrument, for C₂ elimination from C₆₀⁺. The two sets of data are modeled by finite heat bath theory (FHBT) using calculated rate energy dependencies. Calculations are performed using currently available information on the activation parameters. The dissociation is modeled with an Arrhenius pre-exponential factor, A = 5 × 10¹⁹ s⁻¹ (at a temperature K_BT = 0.2 eV) and a vaporization (activation) energy ΔE_vap = 10.3 eV. Modeling the data necessitates inclusion of radiative decay in parallel with the dissociation. Radiative decay is dominant at times longer than a few microseconds.