Absolute cross section for the formation of C⁺ ₆₀ ions produced by electron impact on C₆₀

We employed the fast-neutral-beam technique in a measurement of the absolute partial cross section for the formation of C⁺₆₀ ions following controlled electron-impact ionization of the fullerene C₆₀ from threshold to 200 eV. Our measurement is the first report of an absolute measurement of this cross section which does not employ any normalization procedure and which does not rely on the use of controversial vapour pressure versus temperature data for C₆₀ to determine the C₆₀ number density in the ion source. All quantities that determine the C⁺₆₀ ionization cross section were measured directly in our apparatus. We found a peak value for the C⁺₆₀ parent ionization cross section of 24.4 x 10^-16 cm² in the energy range between 70 and 80 eV. This value is in very good agreement (to within 5%) with the cross section reported earlier by Matt et al (1996 J. Chem. Phys. 105 1880) who employed a novel normalization procedure. Our cross section is significantly lower than the cross sections reported by several other investigators who relied on C₆₀ vapour pressure data in their absolute calibration.