A New Method for Investigating Atomic Electron Velocities

Abstract

When electrons of sufficient speed pass through helium under conditions favorable to single scattering, the electrons scattered through a suitable angle fall into two distinct classes, those scattered elastically and those scattered inelastically. The former have been scattered by nuclei and the latter by atomic electrons. Because the atomic electrons are in random motion, those electrons which have been scattered by them through a definite angle have a distribution of energies, the most probable energy being that corresponding to a collision with an atomic electron at rest. Jauncey has shown that when a fast electron of energy $V_0$ collides with an atomic electron having a component velocity $u$ in a certain direction, the electron will have energy given by $V=V_0{cos}^2\theta +u{\left(\frac{2m{V}_0}{e}\right)}^{\frac{1}{2}}sin\theta$, where $\theta$ is the angle of scattering. It can be shown to follow from this relation that the distribution of energy among the scattered electrons is identical with the distribution of component velocities among the atomic electrons. Moreover, since the last mentioned distribution is closely related to, and identical in shape with, the profile of the Compton modified band in x-ray scattering, measurements of the energy distribution of the scattered electrons will give an experimental determination of the profile of the band. Wave mechanical computations lead to a definite shape for this profile which can now be tested by experiments on electron scattering. A beam of electrons, with energies between 1000 and 4000 volts, was directed into helium at a low pressure and the distribution of energies of electrons scattered at 34.2° measured. It was found that the experimental results gave a profile for the Compton modified band in excellent agreement with the profiles calculated by Hicks and in good agreement with those calculated by Kirkpatrick, Ross and Ritland. Values for the probability of the various component velocities of the atomic electrons are tabulated.