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.
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.
Abstract The emission of polarised light by fluorescent sodium and iodine vapours was discovered by one of the present writers many years ago, and a similar phenomenon was looked for repeatedly, but without success, in the case of the much simpler (presumably) type of emission which occurs when a gas or vapour re-emits without change of wave-length the energy which it abstracts from the incident beam, as when the monochromatic radiation 2536 from a mercury arc traverse as exhausted quartz bulb containing a drop of mercury at room temperature, or the light of a sodium flame falls upon a glass bulb containing highly rarefied sodium vapour. Lord Rayleigh has recently discovered, however, that the resonance radiation of mercury vapour shows fairly strong polarisation, and reported results which apparently indicated that the polarisation becomes stronger as the exciting radiation penetrates to greater and greater depths of the vapour, being practically absent at the point where the radiation enters the bulb or tube. This would mean that polarisation of the light results only when the excitation is, by frequencies, a little out of tune with the natural frequency of the mercury atom, a matter of great theoretical interest. In a report of some work done last year by one of us this last observation of Lord Rayleigh was not confirmed, the polarisation appearing of full strength right up to the entrance window of the exciting beam. A possible explanation of Lord Rayleigh's results will be offered in the present paper in which it will be shown that a very weak magnetic field, properly oriented, destroys completely all traces of polarisation.
Abstract As I have shown in numerous publications, the vapour of mercury in vacuo, even at temperatures below the solidifying point of the metal, when illuminated by the light of a water-cooled quartz mercury lamp, emits the monochromatic radiation of wave-length 2536. According to Bohr’s theory, this resonance radiation results from the fall of an electron from the 2p2 orbit, to which it has been raised from the IS orbit by the absorption of one quantum of 2536 radiation, back to the IS orbit, the orbit occupied by the electron when the atom is in its normal state. An atom in which the electron has been brought to the 2p2 orbit is in a condition to absorb the radiation of any other line produced by the fall of electrons from outer orbits to orbit 2p2, and Fuchtbauer has shown, in an extremely interesting and important paper (‘Physik. Zeit.,’ 1920), that a large number of the arc lines of mercury are radiated by cold mercury vapour illuminated by the mercury arc. He showed that, if the 2536 radiation was screened off by glass, none of the other lines appeared, and explained their presence as the result of the absorption by excited atoms with electrons at the 2p2 level, of the radiations terminating at this level, by which the electrons are carried out to other-orbits, from which they can fall back in various ways, giving rise to emission lines in the process. Though Fuchtbauer‘s experiment did not prove that this process actually took place, his explanation was the only one possible on modern theory, and the experiments which are to be considered in the present paper show that it was correct. Twenty years ago the most that could have been said would have been that the absorption of the 2536 radiation “shook” the atom in some way, causing it to emit its other frequencies as a sort of fluorescence, since Fuchtbauer’s experiment did not show that excitation by lines other than the 2536 line was necessary for the emission of the arc spectrum. To demonstrate experimentally that the step-by-step transfer of electrons from orbit to orbit actually takes place, it is necessary to illuminate the mercury vapour with various combinations of monochromatic light, and study the resultant emission spectrum.
Polarized radiation following electron impact.---If a unidirectional beam of electrons, of velocity slightly in excess of the resonance potential, is projected through a vapor at a very low pressure so that effects due to atomic collision and resonance radiation are negligible, and in the absence of an applied field and resulting Larmor precession, there is reason to suspect that the radiation emitted perpendicular to the electron beam should be partially polarized, especially for lines corresponding to $\ensuremath{\Delta}j=1$.Polarization in mercury and sodium vapor.---Experiment showed a polarization of about 30 percent for $\ensuremath{\lambda}2537$ with the greatest intensity of the electrical vibration having a direction perpendicular to the electron beam. No polarization was detected for the unresolved sodium D-lines, confirming the recent observation of Kossel and Gerthsen.Polarization in mercury with field of 3 gauss.---A field parallel to the electron beam produced no effect. With the field perpendicular to the beam the radiation along the field was unpolarized while that perpendicular to the field was reduced in polarization to 13 percent. These results suggest that the atom radiates with the same distribution of polarization and intensity as in zero field but with the atom rotated through an angle determined by the time of excitation and the precessional velocity.
Abstract In his first papers on the resonance radiation of mercury and sodium Wood describes how the character of the emission changes with increasing pressure of the fluorescent vapour. At low pressure (p = 10-3 mm) the emission comes wholly from a narrow well-defined beam corresponding to the path of the primary light (beam fluorescence). With increasing density of the vapour the beam becomes diffuse until the luminosity fills the whole volume (volume fluorescence) and finally at pressures of about 0·1 mm it withdraws entirely to the spot of entrance of the primary light (surface fluorescence). In all three cases the absorption and emission of light are due to normal atoms; the phenomenon is that of normal or true resonance radiation. For the D-lines of sodium, Lord Rayleigh showed as early as 1919 that, even at pressures at which the true resonance radiation is concentrated on the surface, an emission can in addition be observed within the volume along the path of the primary light, if the D-lines in the spectrum of the exciting source are not too narrow. This new fluorescence beam which can only be excited by the outer parts of the D-lines is partially polarized. Later Jablonsky and Pringsheim found that though the normal resonance radiation observed on the surface is unpolarized under these conditions, the degree of polarization of the new fluorescence beam can be as high as 30% and is practically unaltered by the action of a magnetic field. For the mercury resonance line 2537 A it was Rayleigh who again studied the phenomena at higher pressures but with his main interest concentrated on the duration of the processes and on the appearance of the well-known mercury bands. In his paper, the continuous change of the fluorescence in the vapour volume is studied from room temperature up to 200°C. He used as a source of excitation a “not fully cooled” mercury lamp, giving a somewhat broadened and partially self-reversed line at 2537 A, which produces the effect called by Rayleigh “core effect”. The latter is in Rayleigh’s terminology the fluorescence of mercury vapour produced by the more central part of the 2537 A line while the very different fluorescence emission which is excited by a much broadened and self-reversed line is called by him the wing effect. The core effect always shows a line which, with the available dispersion, seems to coincide with the resonance line itself; when the pressure increases, a number of bands appear which have no importance for the present paper. Since in saturated vapour at 100° C the normal resonance radiation has withdrawn to the surface, it is improbable that the core effect is produced by the most central region of the primary line by which the normal resonance radiation at the lowest pressures is excited.
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.
The polarisation of spectrally resolved Na D lines excited by white, linearly polarised light in a homogeneous magnetic field up to 1000 G was measured by means of the crossed-beam technique. The axis of observation, the sodium beam and the direction of the magnetic field were mutually at right angles. The incoming light beam was directed along the magnetic field and linearly polarised either parallel to the axis of observation or perpendicular to it. A Lyot filter of high transmission and a sensitive photon-counting technique made it possible to detect the small intensities of the resolved Na D doublet excited by the light of a xenon arc lamp. The measured polarisations are found to be in excellent agreement with those calculated by means of Breit's theory and the ambiguities of earlier investigations are completely removed. The importance of our results for electron impact excitation in magnetic fields parallel to the beam is discussed. It is shown that the polarisation of the electron impact excited Na D lines is no longer perturbed by hyperfine structure if a magnetic field of 500 G parallel to the beam is applied.