Energy spectrum of electrons produced by electron impact ionization of helium, nitrogen and oxygen molecules

An expression based on Born-Bethe and Moller formulas is proposed for calculating the ionization cross-section per unit energy of secondary electrons produced by electron impact ionization of the atmospheric gases He, N2, and O2 in the incident energy range 50-2000 eV. The expression requires values of optical oscillator strength, which are available for a large number of targets from absorption data. The results obtained are in good agreement with the experimental data obtained by Opal et al. (1971) for impact energy not less than 100 eV as well as with Born's calculations carried out using generalized oscillator strength for impact energy not less than 300 eV.