The analysis of surface layers by electron diffraction

The very close correspondence which has been shown to exist between the diffraction patterns formed by cathode rays passing through thin solid films,* and the crystal structure of these films, suggests the possibility of using electron diffraction to investigate surface layers of unknown composition. This possibility was indeed indicated in Davisson and Germer’s original paper and has since been further applied. These experiments have all been made with slow electrons, of energies of the order of 300 volts. With such electrons the experiments do not agree well with theory even in the case of known structures, so their application to the investigation of unknown structures involves considerable uncertainty. Further, slow electrons can only be detected photographically with very long exposures, while the electric method of detection is very cumbrous if it is desired to survey the complete diffraction pattern. For these reasons I decided to use the apparatus described in the previous paper to investigate the diffraction patterns obtained by the reflection of cathode rays from the surfaces of various solids. The discharge was generally produced by an induction coil and the energy of the rays was of the order of 30,000 volts. In a few cases, an Evershed and Vignolles direct-current generator was used giving about 6,000 volts. In the course of the investigations, it appeared that these fast rays are uninfluenced by the thin layers of gas which are normally present on surfaces in a vacuum, or, possibly, that they temporally remove the layers by bombardment. From one point of view this is an advantage, as it is therefore unnecessary to take special precautions to degas the surfaces used, or to keep a very high vacuum. On the other hand, it limits the range of the method to the investigation of solid layers.