Angular Distributions of Secondary Electrons in the Dipole Approximation

A close relationship between the angular distributions of secondary electrons produced by photoionization and by impact of fast charged particles is pointed out. The dipole-interaction term in the charged-particle impact cross section [which has $\frac{\left(\ln T\right)}{T}$ dependence, $T$ being proportional to the incident energy] has essentially the same angular dependence as the photoelectrons ejected by unpolarized light. An analysis of recent electron-impact data on He and ${\mathrm{N}}_2$ indicates consistency with the present theory.