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 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 In a previous paper on this subject I examined in some detail the fluorescence excited by the atomic resonance line 2537, and by frequencies lower than this, which act by absorption in the region of longer wave-length than 2537. The fact that this absorption-region is immediately juxtaposed to the atomic line, and that the absorption is (at any rate on a rough view) stronger the nearer we go to the line, makes the distinction between the two cases difficult ; and it must be admitted that the phenomena themselves are not at first sight so distinctive as might be expected, if we take the obvious view that in one case absorption is by atoms and in the other by molecules. This has led some writers to the opinion that atomic absorption does not act at all in giving rise to band-fluorescence. It was shown, however, in the communication mentioned that a discontinuity of intensity in the fluorescence can be observed as the exciting beam traverses the vapour, and is robbed of its more absorbable component. The initial fluorescence due to the core of the resonance line is referred to as the “core effect,” and the much weaker fluorescence distinguishable after the core effect has died out is called the “wing effect,” because it is excited by the wing, or outer region, of the line. With dense vapour, however, excitation can be observed more than 1000 A. to the long wave side of the line, and the term “wing effect” is extended to the whole of this region, because no reason has been found for restricting it. It was found (loc. cit., p. 9) that the core effect is cut out by a supplementary absorption cell of mercury vapour producing a reversal of the line only 0.1 A. in breadth. I wish now to emphasise that this reversal agrees exactly in position with the narrow atomic emission line from a cold source within less than this amount.
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 It is known that the fluorescence of mercury vapour can be excited by absorption in the region λ 1850, situated near the resonance line 1 1S0- 1 1P1, and in the region of band absorption λ 2345. It can also be excited by absorption in the region of the resonance line 1 1S0— 2 3P1 2536.52. In this latter case it has, I believe, been generally supposed that fluorescence is not obtained unless excitation is fairly close to the resonance line. Thus Prings-heim and Terenin found that a hot mercury lamp (punktlampe of Heraeus) or an overloaded ordinary 220-volt mercury lamp does not give band fluorescence of any kind. In the hot lamp the core of the resonance line is removed by self reversal. It is, however, well known that unexcited mercury vapour has an absorption band beginning near the resonance line 2537, and extending, if the vapour is dense, to a distance of some hundreds of Ångstroms towards longer waves. The question arises whether this absorption is accompanied by fluorescent emission or not.
Abstract Prof. R. W. Wood has made many interesting observations on the fluorescence of sodium vapour. They are conveniently summarised in his ‘Physical Optics’ (MacMillan, 1911). The fluorescent spectra he has obtained are apparently connected with the banded absorption spectrum of dense sodium vapour. If white light is employed, this banded absorption spectrum is re-emitted completely as a fluorescent emission spectrum. If monochromatic light is used, a portion only of the complete band spectrum is emitted, this portion consisting of a moderate number of lines approximately equally spaced along a normal spectrum, and including a line coincident with the exciting line. These observations on the band spectrum are of great interest and importance, but observations on the line spectrum of sodium, in absorption and in fluorescence, are more within the range of theoretical discussion at the present time.
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.
Effect of a magnetic field on the polarization of resonance radiation of mercury and of sodium.---(1) Mercury $\ensuremath{\lambda}2536$. A beam of plane polarized monochromatic light was focussed on the bulb containing the vapor at low pressure (0\ifmmode^\circ\else\textdegree\fi{}C), and the polarization of the resonance radiation was determined photographically with the aid of a quartz wedge and a double image quartz prism. If the exciting beam is going east with the electric vector vertical, when the earth's field is carefully neutralized the polarization is 90 per cent in any horizontal direction and zero vertically; a field of only 2 gauss directed north reduces the polarization in its direction to nearly zero (the decrease with increasing field being exponential), changes the polarization east and also up to 60 percent, and increases the intensity directed upwards threefold. Similar effects are produced by fields in other directions. The polarization with zero field does not approach 100 per cent at low pressures, since 90 per cent was found also with the vapor at -50\ifmmode^\circ\else\textdegree\fi{}C, when the radiation first appeared. It was thought that the high value might be due to an orientation of the resonating atom by the field of the exciting light, but a beam of concentrated sunlight produced no effect. As found by Malinowski, a strong field of 10,000 gauss merely increased the intensity of the radiation about 10 per cent. This added light was found to be unpolarized. (2) Sodium D line. In the case of sodium vapor the tube was heated to 185\ifmmode^\circ\else\textdegree\fi{}C, the observations were visual, and as a source of unreversed light, the glow on the surface of a sodium glass vacuum tube carrying a discharge, was used. The effects observed are similar to those for mercury, but differ for some directions; fields forty times stronger were required, and the polarization in zero field was only 6 per cent, probably due to traces of hydrogen. This was increased by the field in some directions to 30 per cent.Effect of magnetic field on fluorescent light of iodine vapor and on white scattered light from mercury and ether vapors.---The polarization was found not to be affected.