Electron Impact Ionization of C60

Absolute electron-impact ionization cross sections for C60 measured by several groups show dramatic differences in the cross section shapes and in the absolute cross section values. The most comprehensive data set of Matt et al. (J. Chem. Phys. 105 (1996) 1880) shows a C60+ ionization cross section which is smaller than the values reported by other groups by a factor of 3. All cross section measurements to date either rely on a determination of the C60 number density in the ion source from vapor pressure data (known to be unreliable for C60) or use a normalization procedure which involves several benchmark cross sections (not all known very accurately). We report a C60+ ionization cross section measure- ment using a fast-beam technique in which all quantities that determine the cross section are measured (no normalization required) and in which the C60 density in the ion source is determined from the energy deposited by a 3 kV C60 beam into a calibrated pyroelectric crystal (no C60 vapor pressure data needed). Our C60+ cross section agrees with the C60+ cross section of Matt et al.