The hyper-fine structure of the arc spectrum, and the nuclear rotation of indium

Abstract

The are spectrum of indium was investigated with the object of finding whether any of its lines possessed hyperfine structure, resulting from a quantized nuclear spin, and a corresponding magnetic moment of the nucleus. The spectrum of cæsium, which was investigated by the author, closely resembles that of indium, both spectra being peculiarly suitable for investigation in respect of hyperfine structure on account of their remarkable simplicity. In the case of cæsium it was found possible to resolve the structure arising from the S-levels, but not that due to the P-levels. In indium, however, it was hoped, on account of the far greater separation of the P-levels and the correspondingly greater interaction between the nuclear spin and the electron orbit, to achieve a resolution of the structure due to the P-levels. This was found possible. The apparatus used for high resolving power was a reflection echelon grating; the instrument was made of fused silica, platinized. It had 25 plates each 7 mm. thick. The grating was made by Adam Hilger, Ltd., and possessed its theoretical resolving power of about 800,000 at a wave-length of 4500 A. U. The mounting and method of using the echelon grating are fully described. The source of light was a cooled vacuum tube containing helium and indium chloride, excited with external electrodes by means of a high-frequency alternating current. The analysis of the structure of the lines shows that the nucleus must be assumed to possess one quantum of rotation. The comparison of the deduced structures of the P ½ and the P 3/2 levels agrees quantitatively with Fermi’s theory of the interaction of the nuclear and the electron spins.