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 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 In 1874 it was shown by Roscoe and Schuster that channelled absorption spectra can be obtained with the vapours of the alkali elements sodium and potassium, and later on these spectra were investigated in some detail by Liveing and Dewar. It was also shown in 1896 by Weidemann and Schmidt that the vapours of these same metals emitted a radiation possessing characteristics of a fluorescence spectrum when they were traversed by white light. Since 1903 exhaustive studies have been made of both the fluorescence and the channelled absorption spectrum of sodium by R. W. Wood, together with a number of collaborators, Including J. H. Moore and F. E. Hackett. In these investigation it was shown that the channelled absorption spectrum of sodium was made up of a number of series of absorption bands, one set of series being on the red wave-length side of the D lines, and another lying in the visible blue-green region. In addition, series of absorption bands were found by them with approximately regular spacing in the neighbourhood of λ = 3303 A, the second member of the doublet series of this element. As regards the fluorescence spectrum of sodium, they found that, by stimulation of the vapour with approximately monochromatic light, there resulted an emission of light, the spectrum of which consisted of a number of bright but narrow bands of varying intensity, more or less regularly spaced both above and below the mean wave-length of the exciting light. They observed, too, that the slightest change in the wave-length of the exciting light resulted in the disappearance of one set of lines and in the appearance of another of different wave-lengths. In the various florescence spectra obtained by R. W. Wood when stimulating sodium vapour by monochromatic light from different sources, it was noted that there was a remarkable recurrence of the interval, ∆λ = 52·3 A. in the spacing of the fluorescence bands.
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 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.
TPHE purpose of this investigation was to study the resonance spec--*-trum of sodium vapor when the resonance was excited by a single D line.In a paper on " Resonance Radiation of Sodium Vapor "* published by one of us in 1905, it was shown that if a bulb containing pure sodium vapor was illuminated by light from a sodium flame, the vapor emitted a yellow light which spectroscopic analysis showed to be identical with the exciting light, in other words, the two D lines.It was suggested at this time that it would be interesting to see if both D lines appeared when the vapor was illuminated by a single D line.This would determine whether the mechanisms that give rise to the D lines are separate or in some way connected.In 1914 this experiment was tried by Wood and Dunoyer. 2 This experiment was made possible by the discovery of a polarization method of separating close spectral doublets that eliminated the great loss of light involved in a high-power monochromator, but even with this method the resonance light is very faint.The spectrum of resonance excited by D 2 was photographed with exposures varying from 10 to 15 hours.The majority of the plates showed only the D 2 line, but owing to under exposure Di would not have been visible if it was less than one fourth of the intensity of D 2 .Some plates indeed showed both Di and D 2 , but the presence of Di in the exciting light was judged to be the cause of this.Imperfections in the optical system made it impossible to entirely remove Di from the exciting light and as it was sensitive to temperature changes the nature of the transmitted light sometimes changed during an exposure.It was concluded that sodium resonance excited by D 2 consisted of D 2 alone, but the results admit of two other possibilities.Di light may always be present though much fainter than D 2 , or it may only appear under certain conditions.Other resonance phenomena show that the radiation centers in sodium are not entirely independent.One of us 3 showed in 1905 that excitation 1
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.