Appearance Energies Of C-60 Fragment Ions Revisited

Following up on a serious discrepancy (factor of 2) in reported appearance energies for fragment ions produced by electron impact ionization of C-60, we have measured here the appearance energies as a function of electron current using a crossed beam apparatus dedicated to ionization cross section measurements. Thus, it was possible to conclude that the low-energy peaks in the electron ionization curves of Baba et al, (which have been used to deduce the appearance energies) are very likely caused by consecutive ionization and excitation processes due to the high electron currents employed in these measurements, From this it follows that the true appearance energies are the high appearance energies reported earlier by Mark and co-workers (and Anderson and co-workers) which were determined at low ionizing electron currents to prevent secondary processes in the ion source, Moreover, rye discuss here the crucial role of the kinetic shift in deriving activation energies and other thermochemical properties from measured appearance energies.