The elastic scattering of slow electrons in gases.—II

Abstract

In a previous paper results of measurements of the angular distributions of slow electrons scattered elastically in argon were described. The appearance of maxima and minima in the curves obtained suggested the analogy of the scattering of electrons by atoms to the scattering of light by small spheres. This idea was given further credence by the results obtained by Arnot for the angular distributions of electrons scattered elastically in mercury vapour. However, a number of important and interesting phenomena occur in the scattering of electrons by atoms which are not present in the scattering of light by small spheres. Thus there is no optical analogy to the distortion of the de Broglie waves of the electrons by the potential field of the atom, nor to the exchange of electrons between the atoms and the colliding beam. The relative importance of such phenomena in determining the scattering of slow electrons will depend on the properties of the scattering atoms, so we may expect to be able to sort out the individual effects by comparing the results obtained with different atoms in the light of available theory. In order to do this the previous angular distribution measurements have been extended to helium, neon, nitrogen, hydrogen and methane, and as a consequence the interpretation of the phenomena has been much clarified. In this paper the results obtained in the above-mentioned gases are described and discussed. 2. Method and Results . The apparatus used was, except for minor modifications, the same as that described in paper I. Electrons from a tungsten filament were accelerated through a pair of slits by a suitable potential giving a homogeneous beam of known energy. This beam was fired through the gas under investigation at low pressure (1⋅5 X 10 -3 mm. Hg) in a field free space and the scattered electrons collected by a Faraday cylinder. This cylinder could be rotated by a ground joint so enabling the intensity of the scattered beam to be measured at different angles of scattering.