Inelastic Collisions of Electrons in Helium and Townsend's Ionization Coefficient

Abstract

It is pointed out that the accuracy of the cross sections observed by Maier-Leibniz for the excitation of helium by slow electrons may be tested by using them to calculate the Townsend ionization coefficients α and comparing the results of such calculations with observed values. Information about the reliability of the difficult measurements of excitation cross sections is essential to check attempts made to develop a satisfactory theory. To carry out this test it is necessary to extend and to some extent refine the work of Smit on the velocity distribution of electrons in helium diffusing under the action of a uniform electric field F when inelastic collisions are allowed for. This has been done and numerical results have been obtained for F/p=10 and 20 v/cm/mm Hg. The ionization coefficient has been measured for the higher values of F/p and agrees quite well with that calculated when Maier-Leibniz's cross sections are used. The sensitivity of this test of the cross sections was examined by calculating α assuming that Maier-Leibniz's values were respectively twice and one-half the correct value. It was found that the calculated value of α is nearly proportional to the size of the assumed excitation cross section (it was assumed throughout that the ionization cross section is given correctly from Smit's observations). It is concluded that the observations of Maier-Leibniz may be used for checking theoretical determination of excitation cross sections in helium.