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 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 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 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 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.
In the presence of heavy foreign gases, such as Kr and Xe, diffused absorption bands were observed at both the short wavelength and the long wavelength side of the first four absorption doublets of the Rb principal series. The red bands were observable when the Rb vapor pressure in the absorption tube was relatively low (10−3 to 10−2 mm Hg). The violet bands were not intense enough until the Rb vapor pressure was increased to 0.1–1 mm Hg. With other lighter gases (H2, He, Ne, A, N2) only violet bands were observed. The positions and the absorption contours of the bands in relation to the member of the absorption series and to the nature of the foreign gases are discussed.
We describe a series of measurements of absorption and laser induced fluorescence on cells that contained cesium and rubidium and a rare gas: He, Ar, Kr, or Xe. These studies showed strong blue wing absorption to the short wavelength side of the alkali atom D2 lines due to collisionally formed Cs- or Rb-rare gas excimers. We also have observed an efficient two photon excitation of higher lying states in Cs and Rb that produces both intense blue emission and IR atomic emission in the 1.3 to 3.8 μm spectral region.
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.