APPEARANCE AND IONIZATION ENERGIES OF SINGLY, DOUBLY AND TRIPLY CHARGED C-60 AND ITS FRAGMENT IONS PRODUCED BY ELECTRON-IMPACT IONIZATION

Electron impact ionization and dissociation of C-60 has been studied as a function of electron energy using a crossed molecular beam/electron beam double-focussing mass spectrometer system. Appearance energies have been determined (using a special threshold extrapolation procedure) for singly, doubly and triply charged parent ions C-60(z+) and for singly, doubly and triply charged fragment ions C-60-2m(z+) with m ranging from 1 to 8. The first three appearance energies of C-60 with values of 7.6 +/- 0.5, 19.0 +/- 0.5, and 35.6 +/- 1.0 eV, respectively, are in excellent agreement with reliable data reported previously. Further conclusions about the dissociation mechanism are derived from the result that the difference in appearance energies for consecutive fragment ions of C-60 is nearly constant and independent of the charge state (yielding an average value 5.7 +/- 1.3 eV) and the ionization mechanism.