Appearance potential and electron impact ionisation cross-section of C₆₀

Abstract

A Knudsen cell mass spectrometric technique was employed to determine the electron impact ionisation cross-section of C₆₀. Ion intensities were measured as a function of time, and weight loss due to evaporation was determined. Similar experiments were carried out with silver to obtain calibration factors. From the weight loss and the ion intensity integrated over the entire duration of the experiment for both siver and C₆₀, the ratio of the ionisation cross-section of C₆₀ to that of silver was obtained. Taking the value for silver from the literature, the ionisation cross-section of C₆₀ at 38 eV was calculated to be (53.5 ± 5.6) × 10^-16 cm². This experimental value (determined for the first time) is discussed in relation to those derived by using various empirical formulae generally applied for obtaining molecular cross-sections. Ion intensities were measured as a function of electron energy to obtain ionisation efficiency curves for C₆₀, and its appearance potential was determined by linear extrapolation of these curves.