The arc spectrum of silicon

In the Bakerian Lecture for 1924 it was shown that the changes in the spectrum of silicon which accompany increased energy of excitation represent successive stages of ionisation of the silicon atom. Besides the spectrum of the neutral atom (Si I), characterised by the series constant R, three other spectra were identified as Si II, Si III and Si IV, for which the series constant was 4R, 9R and 16R respectively, in accordance with Bohr’s theory. In view of the great advances in the theoretical interpretation of spectra which have since been made, it has seemed desirable to undertake a revision and extension of the earlier analyses of these spectra. The present communication gives an account of additional observational data for Si I and of the progress which has been made in the identification of the terms predicted for this spectrum by the theory of Heisenberg and Hund.