IONIZATION OF C-60 (BUCKMINSTERFULLERENE) BY ELECTRON-IMPACT

Utilizing a crossed electron-beam/molecular-beam collision geometry, m ass spectra of C60 and C70 ions have been obtained for an electron imp act energy range of 20 eV to 600 eV. These spectra are dominated by si ngly and doubly ionized species. The intensity of fragment ions is ver y weak, which is in agreement with previous observations. Ionization e fficiency curves for the formation of C60+ and C60(2+) have also been obtained near their threshold regions. From these curves, appearance e nergies (AE) of these ions have been derived. The AE for C60+ is found to be 7.8+/-0.3 eV, which agrees well with previous measurements. How ever, our value of 16.4+/-0.3 eV for the formation of C60(2+) is much lower than the value of 19.0+/-0.03 eV obtained by photo-ionization te chniques.