The spectrum of H 2 : The bands analogous to the ortho-helium line spectrum

Abstract

In “Structure in the Secondary Spectrum of Hydrogen—Part V” it was shown that there existed in this spectrum a very extensive series of bands whose null lines were related by a Rydberg-Ritz formula and whose electronic terms were very close to those of the principal series of the so-called helium doublets (orthohelium) showing that the spectrum of H₂ is closely analogous to the line spectrum of He. [It is also very similar to the spectrum of He2.] This similarity has since been confirmed by the discovery and investigation of the absorption spectrum of unexcited H₂ by Dieke and Hopfield from which it appears that none of the final 2 states of that spectrum, in which the transitions are from 1 ¹S to an upper 2 state, are the same as the final 2 states of these emission bands. On the other hand their 2 ¹S states do agree, to the accuracy of the ultra-violet data, with the final states of an entirely different set of band systems in the visible spectrum which are therefore analogous to the parhelium spectrum. The conclusions of Part V were drawn from a study of the Q branches of the bands only. In an earlier paper (Part IV) a preliminary investigation of some of the accompanying P and R branches had been made by a study of the intensity distribution in these bands in the first type discharge. This method enables the lowest rotational quantum number lines of the bands to be picked out with some certainty but the upper lines cannot in general be recognised on account of the faintness of the discharges. These can only be located by finding lines which satisfy some reasonable combination principle. An attempt made by one of us (O. W. R.) in collaboration with Dr. D. B. Deodhar to extend the classification of Part IV led us to the conclusion that the existing data were inadequate, mainly owing to insufficient resolution, to enable a satisfactory decision to be formed as to whether the details of Part IV were correct or ought to be modified.