Appearance and ionization energies of multiply-charged C70 parent ions produced by electron impact ionization

Using an improved crossed beams/mass spectrometer apparatus we have extended previous electron impact ionization studies concerning ionization cross sections and appearance energies of up to quadruply-charged ions of C70 to ions with charge states up to 6. A novel refined data procedure, involving a simultaneous non-linear weighted least-squares fit of two functions, allows the extraction of the cross section threshold. The ionization energies obtained depend linearly on the precursor charge state and compare well with theoretical predictions and the molecular capacitor scaling law. The corresponding maximum cross sections of C705+ and C706+ are about 1.5 × 10−18 and 1 × 10−20 cm2, respectively.