Abstract Anomalous dispersion in the region of the red lines of potassium was first observed by Ebert in 1904. The method adopted for Ebert’s experiments was a modified form of the crossed prism method used by Wood in the investigation of the corresponding phenomena in the case of sodium. Potassium was heated in a tube through which two currents of hydrogen passed from each end to a central outlet. The cool hydrogen kept the potassium vapour in a prismatic form, so that light passing along the length of the tube suffered deviation and dispersion by the potassium vapour prism. The author of the present communication has shown that there is no need for the hydrogen streams. If the tube be kept cool on its upper surface the metallic vapour takes of itself a prismatic form or is arranged in layers of decreasing density, and so behaves in a similar way to a prism of homogeneous vapour. The present communication deals with quantitative results from the measurement of dispersions at different wave-lengths, and it appears that the deviation due to potassium vapour is observable over the whole of the visible spectrum and for a considerable distance in the ultra-violet. Strong absorption takes place at the lines of the principal series and for wave-lengths near these series lines we have "anomalous" dispersion. This phenomenon has been observed at seven of the pairs forming the principal series lines for potassium—as the pairs of lines in this series get closer and closer together with diminishing wave-length, the dispersion effects after the first two pairs are only observable outside the lines forming a pair, but there appears a lack of symmetry in the observed dispersion curves corresponding to the different intensities of the lines forming the pair. The dispersion to be observed may then be regarded as that corresponding to the principal series absorption lines; no other absorption region seems to affect the dispersion —at any rate at low densities of vapour.
Abstract Anomalous dispersion in the region of the red lines of potassium was first observed by Ebert in 1904. The method adopted for Ebert’s experiments was a modified form of the crossed prism method used by Wood in the investigation of the corresponding phenomena in the case of sodium. Potassium was heated in a tube through which two currents of hydrogen passed from each end to a central outlet. The cool hydrogen kept the potassium vapour in a prismatic form, so that light passing along the length of the tube suffered deviation and dispersion by the potassium vapour prism. The author of the present communication has shown that there is no need for the hydrogen streams. If the tube be kept cool on its upper surface the metallic vapour takes of itself a prismatic form or is arranged in layers of decreasing density, and so behaves in a similar way to a prism of homogeneous vapour. The present communication deals with quantitative results from the measurement of dispersions at different wave-lengths, and it appears that the deviation due to potassium vapour is observable over the whole of the visible spectrum and for a considerable distance in the ultra-violet. Strong absorption takes place at the lines of the principal series and for wave-lengths near these series lines we have "anomalous" dispersion. This phenomenon has been observed at seven of the pairs forming the principal series lines for potassium—as the pairs of lines in this series get closer and closer together with diminishing wave-length, the dispersion effects after the first two pairs are only observable outside the lines forming a pair, but there appears a lack of symmetry in the observed dispersion curves corresponding to the different intensities of the lines forming the pair. The dispersion to be observed may then be regarded as that corresponding to the principal series absorption lines; no other absorption region seems to affect the dispersion —at any rate at low densities of vapour.
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 A region of continuous absorption lying on the short wave-length side of the limit of the principal series and corresponding to the photo-ionization of the atom is to be expected in the alkali metal vapours. The currents due to photo-ionization have been measured and several attempts have been made to observe the absorption. The main difficulty is in the action of the vapour on the windows of the absorption cell. This has usually been overcome by keeping the ends of the absorption tube cool and checking the diffusion of the vapour from the centre of the tube by means of a foreign gas. This gas produces a large and complicated effect on the absorption (Harrison). In the present experiments it has been found possible to avoid the use of a foreign gas by employing a long absorption tube and a low vapour pressure. Cæsium was used because it gives a sufficient pressure at a moderate temperature and because its series limit lies in the most convenient region of the spectrum.
Abstract The first section of this paper is an account of some experiments on the absorption of light in sodium vapour from the series limit at 2412 Å to about 1600 Å (an energy difference of 2·6 eV). The absorption cross-section at the limit is 11·6 ± 1·2 x 10-20 cm2. The cross-section decreases giving a minimum of 1·3 ± 0·6 x 10-20 cm2 at 1900 Å and then increases to 1600 Å. A theoretical calculation by Seaton based on the dipole-length formula gives good agreement with the experiments at the series limit and also correctly predicts the wave-length for the minimum, but it predicts a significantly lower absorption at the minimum. The experiments described in the first section of the paper conclude a series on the absorption of light in the alkali metals. The second section consists of a general discussion of the results of these experiments and of their relation to theoretical calculations. There is good agreement between theory and experiment except in regard to the magnitude of the absorption at the minimum.
Abstract In two preliminary papers we have recorded the discovery that when monochromatic light is scattered in a transparent medium (be it gas, vapour, liquid, amorphous solid or crystal), the diffused radiation ceases to be monochromatic, and several new lines or sometimes bands (associated in many cases with a continuous spectrum) appear in the spectrograms of the diffused radiation. Further, the new radiations are, in general, strongly polarised. That the phenomenon is entirely distinct from what is usually known as fluorescence is clear from the fact that the effect is observed when both the exciting radiation and the new radiations generated by it are far removed from the characteristic ultra-violet and infra-red frequencies of the medium. As an illustration we may mention the case of transparent crystalline quartz in which the effect is very well shown with the 4358 A. U. line of mercury as the exciting line, the new lines also appearing in the indigo-blue region of the spectrum. Our preliminary studies have proved conclusively that the effect arises in the following way: The incident quantum of radiation is either scattered as a whole, in which case we have the classical scattering, or else is absorbed in part by the molecules of the medium, the remaining part appearing as a scattered quantum. The part absorbed shifts the molecule to a level of energy different from the initial state. The possibility of a process of this kind, in respect of the electronic state of an atom, was first contemplated by Smekal, and figures prominently in the theory of dispersion due to Kramers and Heisenberg, and in the papers of Schrödinger. Our experiments furnish definite proof of the possibility of such processes, and show that they may occur also in such complicated systems as the molecules of a vapour or a liquid or even in a complete crystal. In the series of papers of which this is the first our further studies of the new radiations will be discussed.
A detailed theory of the frequency dependence of the electro-optical Kerr effect of an atom in the region of absorption lines is presented and applied to the particular case of 2 P←2 S doublets of sodium and lithium vapours. Doppler broadening is taken into account. With electric dipole matrix elements derived from the Coulomb approximation for both discrete and continuum states, the molar Kerr constant, m K(32 P←32 S), of atomic sodium vapour in the D-line absorption region is calculated to ±30 per cent; the calculations indicate that m K in the vicinity of the D2 line (32 P 3/2←32 S 1/2) should be very much greater than near the D1 line (32 P 1/2←32 S 1/2) in qualitative agreement with the experiments of Kopfermann and Ladenburg Qualitative consideration is also given to dispersion of magnetic birefringence; in contrast with electric birefringence strong dispersion is expected near both lines of alkali metal 2 P←2 S doublets.
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.