Investigations on the spectrum of selenium.—Part II, Se III

The present paper is a continuation of the work on the spectra of selenium, carried out by the authors partly in Professor Fowler’s Laboratory at the Imperial College and partly in Professor Siegbahn’s Laboratory at Uppsala. A detailed account of the experimental part of the investigation together with the analysis of Se IV and Se V has been given in a previous communication. As already reported in ‘Nature’ the experimental data there described have led to the identification of the system of energy levels characteristic of the spectrum of doubly-ionized selenium. It is the purpose of the present paper to give an account of the structure of this spectrum so far as it is revealed by a study of the spectrograms of selenium under different conditions of excitation. The lines of the spectrum in question appear when discharges, even of moderate intensity, are passed through selenium vapour contained in a capillary tube. In the ordinary spark in air, the spectrum is represented only by a very few of the stronger lines which could be ascribed to Se III, but with strong discharges through capillary tubes, the lines are considerably enhanced and the complete spectrum is easily photographed. In the region of very short wave-lengths, below λ 1500, the source found suitable for the excitation of the spectrum was the spark in vacuum between aluminium poles tipped with metallic selenium. With this source, excited by a 50 K. V. transformer, it was found necessary to include an inductance of about 0·5 m. h. to elicit lines of Se III and Se II. At the same time, a study of the spectra excited with and without inductance in series in the secondary circuit has greatly facilitated the assignment of the lines to the various stages of ionization of the element. Reference may be made to a reproduction of the spectra in the previous paper by the authors (hoc .cit .), where the method of measurement and the instruments employed for photographing the various regions of the spectrum have also been described.