On the Interaction Between Radiation and Electrons

Abstract

In the production of recoil electrons we have an example of the action of radiation on free electrons, whereas the photoelectric effect with x-rays is an example of the action of radiation on a pair of positive and negative charges. In both effects experiment indicates that the whole momentum absorbed from the radiation is imparted to the electron that is set in motion by the radiation, showing that the duration of the action of the radiation is short compared with the natural period of the electron in the atom. It is assumed that the action is sensibly instantaneous. In contrast with the prediction of Lorentz's force equation, which would predict an impulse imparted to an isolated electron almost in the direction of the electric vector, the experiments show that the preferred direction of motion of the recoil electrons is perpendicular to the electric vector. An impulse on a free electron in the direction of the electric vector would not be consistent with the conservation of momentum. The photo-electrons on the other hand have the electric vector of the incident wave as their preferred direction of motion (neglecting radiation pressure), though the experiments show that the impulse imparted to the electron by the radiation may make a considerable angle with the electric vector. In this case the conservation of linear momentum permits motion in any direction, since equal and opposite impulses are applied to the positive and negative parts of the atom by the electric vector; but the conservation of the angular momentum of the system requires that the impulse shall be imparted in a direction determined by the instantaneous position of the electron in the atom. The experiments of Auger and Bubb are consistent with this requirement, but indicate that Lorentz's force equation is only statistically valid in defining the direction of the action of the electric vector on the photo-electron.