The scattering of electrons by metal vapours. I.—cadmium

Abstract

Since the discovery of the diffraction of electrons by gas atoms* a large amount of experimental and theoretical work has been devoted to the study of electron scattering in gases. As a result it is now possible to recognize the main processes occurring in the collisions with the gas atoms and it has been found that it is usually only necessary to calculate the scattering by the undisturbed field of the atom in order to explain the experimental results. As a consequence considerable simplification is introduced in the theory of the phenomena and it follows that the diffraction effects are mainly determined by the ratio of the wave-length of the incident electrons to the distance from the centre of the atom at which the magnitude of the potential energy of the electron in the atomic field is comparable with its kinetic energy. When this ratio is large (very slow electrons) the angular distribution per unit solid angle of the scattered electrons is independent of angle. As the ratio decreases maxima and minima appear and the diffraction effects become more and more complicated until such electron energies are reached that the ratio begins to increase again. For such energies the simple picture fails but Born’s approximation applies and the angular distribution per unit solid angle decreases uniformly with increase of angle. Thus one would expect potassium and argon to give similar angular distributions for electrons with energies considerably greater than the ionization energy of the N electron of potassium but, when the electron energy becomes comparable with this energy, the presence of the outer electron in the alkali metal atom should produce much more complicated angular distributions than are observed for electrons of the same energy scattered by argon. If the above view of the phenomena is correct, it follows that the field of an atom may be approximately determined merely by comparison of the diffraction effects which it produces in scattering electrons with those produced by atoms whose fields are known. All that is necessary is to effect this comparison at a series of different electron energies. A generalization of this method to molecules which have approximately spherically symmetrical fields would also be possible.