Threshold Law for the Probability of Excitation by Electron Impact

Abstract

The threshold law for processes of excitation is difficult to measure directly and lack of experimental evidence has prevented a choice being made between different theoretical treatments. The ionization efficiency curves for the ion-pair processes, I+ from I2, O+ from O2, and C+ from CO, and for what is here claimed to be auto-ionization in N2+ and O2+ have been measured. It is shown that these curves measure excitation probabilities and that the threshold law is in all cases a step function of the energy. The implications of this with regard to the assignment of dissociation products and the measurement of appearance potentials are discussed.