The Structure of the Band Spectrum of Helium. V
Proceedings of the Royal Society A
Introductory. The analysis of this spectrum has now reached a point at which it becomes possible to predict the positions of new bands, sometimes precisely and some times only approximately, with some degree of confidence. There are two distinct and non-combining families of electronic levels which are closely analogous to those of atomic helium,J and each of these is accompanied by one or more relatively weak vibrational levels. The main features of the rotation structure of any particular
... any particular band can also be predicted from a knowledge of the electronic levels involved. I t therefore seemed obvious th a t in continuing this work § attention should first be directed to locating and analysing the predictable bands which might be expected to appear on the spectrograms available. These having been accounted for the analysis of the remainder, if any, of the spectrum should be simplified. Work was accordingly begun on these lines, and a search made for the bands 4D -> 2P of the orthohelium family and for 3D -* 2P and 3S -*• 2P of the parhelium family. I t had not proceeded far, however, when it became apparent th a t the spectrum contained a number of other bands, some of them quite strong, which did not fit into the existing scheme as regards either location or structure. Most of these had been analysed into branches in the early stages (1922) of the investigation of this spectrum, but publication of the results was postponed until some interpretation of them could be obtained. This was now found to be possible with the help of more recent developments, and as the results promised to be of considerable interest it was thought preferable to abandon the original plan and to concentrate on the new bands. The present communication deals with four of these, together with one member of the " ordinary " orthohelium family. t Mulliken, * Proc. Nat. Acad. Sci.,' vol. 12, p. 158 (1926). § The previous papers of this series will be referred to as I, II, III, IV, and have appeared in these '