Abstract The possibility of demonstrating the spin of the electron directly has been discussed since the first growth of wave mechanical conceptions, and the ascription of a magnetic moment to the electron to explain doubling of spectral lines. Bohr has shown, however, that any experiment designed to do this by deflection in a magnetic field, in any manner similar to that used by Gerlach and Stem to measure the magnetic moment of atoms, must fail owing to the operation of the Uncertainty Principle. It does not seem, however, that this will hinder us from demonstrating an asymmetry in the electron by means of scattering experiments; the method is closely analogous to the asymmetric scattering of polarised light, and was in fact used by Barkla to show the polarisation of X-rays. The earlier experiments which were carried out on the polarisation of electrons by Wolf, by Joffé and Arsenieva and by Davisson and Germer all led to negative results. The very careful work of Davisson and Germer should be specially mentioned, in that they reflected electrons from the faces of single crystals at angles corresponding to coherent scattering of the waves. Working up to a velocity of 120 volts they found no asymmetry in the scattering more than ½ per cent., which was within the limit of error of the apparatus. Darwin has shown, however, that no polarisation is to be expected in this type of experiment.
Abstract The possibility of demonstrating the spin of the electron directly has been discussed since the first growth of wave mechanical conceptions, and the ascription of a magnetic moment to the electron to explain doubling of spectral lines. Bohr has shown, however, that any experiment designed to do this by deflection in a magnetic field, in any manner similar to that used by Gerlach and Stem to measure the magnetic moment of atoms, must fail owing to the operation of the Uncertainty Principle. It does not seem, however, that this will hinder us from demonstrating an asymmetry in the electron by means of scattering experiments; the method is closely analogous to the asymmetric scattering of polarised light, and was in fact used by Barkla to show the polarisation of X-rays. The earlier experiments which were carried out on the polarisation of electrons by Wolf, by Joffé and Arsenieva and by Davisson and Germer all led to negative results. The very careful work of Davisson and Germer should be specially mentioned, in that they reflected electrons from the faces of single crystals at angles corresponding to coherent scattering of the waves. Working up to a velocity of 120 volts they found no asymmetry in the scattering more than ½ per cent., which was within the limit of error of the apparatus. Darwin has shown, however, that no polarisation is to be expected in this type of experiment.
Abstract 1. As soon as it was appreciated that the phenomenon of the "spinning electron" was to be attributed to the existance of simultaneous wave functions, the question naturally arose as to how far there would be an analogy with the polarisation of light, and in particular whether in diffraction by a slit or grating or in reflexion from a mirror, electrons would be selected having a preferential direction of magnetisation. Dynamical conceptions suggest that it would require a magnetic force to produce a change in the polarisation, so that the general indication was rather strongly negative, and it seemed hardly worth while to verify the fact by direct soultion. I believe, however, that recently experiments have been undertaken to search for such an effect (by means of successive diffractions from two crystals), and in view of this I have examined the question more closely. The result, broadly speaking, bears out the first conjecture that no effect could be observed in practice, but in certain cases (of a rather hypothetical kind) an effect might be found and the whole problem is interesting as an example of the wave theory. The essence of the matter is sufficiently represented by taking a line grating instead of a crystal, and, furthermore, by limiting the electric and magnetic forces of the grating in such a way that there shall only be first order spectra on each side, two reflected and two transmitted. The grating is supposed made out of periodic distribution of electric or magnetic material. Actual crystals will consist of almost purely electric material, but the magnetic case is as easy to treat and, though unpractical, gives rise to interesting results. It will appear that when a polarised electron wave falls on such a grating, the direction of magnetisation may be changed in the diffracted rays. It is a true polarisation effect in that the change in direction is independent of the strength of the field in the grating, which only determines the total intensity.
Abstract The work about to be described is a continuation of an investigation begun by one of us at Princeton, New Jersey, on the scattering of electrons by gas atoms. The conditions there were so chosen that only single collisions were taken into account, and it was possible to investigate not only the energy loss on collision, as in the Franck and Hertz experiments, but also the direction of the path of the electron after collision. By employing a magnetic analysis of the velocity of the electron after impact it was found possible to obtain information as to the probability of excitation of various stationary states of the atom considered, on collision with electrons of much higher velocities than could previously be used. The curves relating scattered intensity with angle of scattering were found to have sharp maxima, which were strongly suggestive of diffraction maxima, as might be expected if the motion of electrons is governed by the wave mechanics. But whereas the wave mechanics has been eminently successful in explaining the scattering results of Davisson and Germer with single metal crystals, and of Thomson and Rupp with thin metal foils the results of Dymond have been inexplicable on this basis. It may be said at once that we have been unable to find any trace of the sharp maxima found by him in I, which were apparently due to a secondary cause. Indeed, as has recently been pointed out by Elsasser, no interference effects can be expected, when the electron loses energy on collision, the case in which Dymond found maxima, for the frequencies of the electron before and after interaction with the atom are different and there can consequently be no coherence of the scattered waves.
Abstract 1. M. L. de Broglie has introduced a theory of mechanics according to which a moving particle behaves as a group of waves whose velocity and wave-length are governed by the speed and mass of the particle. In fact if m0 is the mass for slow speed and v the speed of a freely moving particle, the wave-length is given by λ = h√1-v2/c2 / m0v, and the wave velocity V by V = c2/v, the group velocity being v, the velocity of the particle. Here c is the velocity of light and it will be seen that the wave velocity is greater than c. There is nothing impossible in this because the waves are regarded as purely geometrical —“phase waves”—not as carrying energy. Compare, in ordinary optical theory, the case of substances, such as sodium, for which the refractive index is less than unity. The above is for free space; in the presence of a field of force V varies, and the consequent bending of the waves by refraction corresponds, on the new theory, to the deviation of the path of the particle by the field of force, on the old. The consequences of this theory have been worked out by de Broglie, Schrödinger and others and applied to problems in spectroscopy where they have provided the solution of several outstanding difficulties left by the older theory of orbits. In view, however, of the extremely fundamental nature of the theory it is highly desirable that it should rest on more direct evidence, and, in particular, that it should be shown capable of predicting as well as of merely explaining. Dymond has obtained some remarkable results on the scattering of slow electrons in helium which are of the general nature to be expected in this theory, but our knowledge of the structure of helium, together with the mathematical difficulties of the problem have so far prevented any exact comparision of the theory with experiment. Davisson and Kunsman and Davisson and Germer have obtained results on the reflection of slow electrons from the surfaces of crystals, especially nickel, which show good qualitative agreement with the theory but a discrepancy of 30 per cent. in certain magnitudes. It is hoped that the experiments described in this paper will advance the matter a stage further. They are a development of some experiments of which a preliminary account appeared recently in ‘Nature.’
Among the secondary effects produced in an X-ray, or cathode ray tube, there is one which is of special interest to physicists because it seems to throw some light on the question as to the nature of an electron. It appears that when a pencil of cathode rays falls upon the surface of a metal crystal, some of the rays are reflected, or “scattered,” and that the intensity of the scattered rays is greatest in certain directions, so that on a photographic plate placed to receive them, a diffraction pattern is produced as with X rays. The experiment proves that electrons, which were till recently supposed to be merely “particles,” really consist (like X rays), of waves, or at any rate have waves associated with them. It is to these waves associated with electrons, or “electron waves” as they are called, that I propose to devote my remarks this evening. The effect was first observed experimentally in 1927, by Davisson and Germer, who reflected cathode rays from a nickel crystal and, using an electrical method of detection, found maxima of intensity in certain directions. From the positions of the maxima and the known spacing constants of nickel they were able to deduce the wave-length of the electron waves. Later in the same year Professor G. P. Thomson obtained similar effects by transmitting cathode rays through thin metal films and receiving them on a photographic plate.
Davisson and Germer performed an experiment, in which electrons were reflected from parallel layers of atoms in a crystal. The electrons showed interference, thus demonstrating the wave nature of the electron. Louis de Broglie postulated a wave model of the electron, in which an electron orbit was represented by a group of in-phase waves. The model predicted that the angular momentum of an electron could take on only a limited number of values, a concept that Neils Bohr had introduced some years earlier as an ad hoc assumption. In an electron microscope, electrons stream off a hot filament and are accelerated to high velocity by passing between two electrically charged plates. The electrons' wavelength is related to their velocity and it is easy to obtain electrons with wavelengths near 0.1 nm, which suggests the possibility of obtaining resolution of about the same magnitude.
Abstract In a number of recent papers descriptions have been given of how the light from atoms excited by a directed stream of electrons is polarised, in the absence of any external field of force. In particular we may refer to the paper by one of the present writers (Paper I), where some of the characteristics of the effect in mercury were discussed. It was found that the different lines of the spectrum were differently polarised, both in magnitude and direction. The behaviour of the polarisation when a magnetic field is applied to the mercury atoms was found to be identical with the effect produced by a similar field on the polarisation when the light is excited by the absorption of plane polarised light, the electric vector of which is parallel to the direction of the stream of electrons. In this way, the polarisation of the light excited by electron impact was shown to be closely related to the polarisation of resonance radiation, which had been investigated by Wood and Ellett and others. In the paper referred to, the measurement of the polarisation of the spectral lines was, except in an individual instance, only qualitative. Since its appearance two papers have appeared in which polarisation measurements of the mercury lines excited by electron impact have appeared. Eldridge and Olsen gave qualitative results, and these confirmed those which we had given. Quarder gives quantitative measurements which are also for the most part in general accord. The aim of the present paper is to describe quantitative results, and, as will be seen, these are not in all cases in detailed agreement with those given by Quarder. The second aim is to determine the variation of the polarisation with the velocity of the electron stream, a point which has not been previously attempted. In Paper I it was shown that the magnitude of the polarisation is intimately connected with the dynamics of the collision process. By the electron impact, the atom is raised from its normal state into an upper quantum state, and by its return into the normal state, or into another state, light is emitted. The magnitude of the polarisation depends on both of these processes, but the second process, the actual emission of the light, is well understood. Polarisation measurements therefore give a powerful means of investigating the collision of an electron with an atom in the case when excitation of the atom takes place, and it is in this that their main interest lies.