The structure of the band spectrum of helium.—IV

Since the publication of the last paper in this series some important contributions to the interpretation of the spectrum have been made, notably by Mulliken. In extension of his work on other bands he has proposed electronic term designations for all the helium bands published up to the present, and has shown that these designations provide an explanation of several characteristic features of the bands, such as the absence of certain branches in some of them, and the number of “missing lines” in the neighbourhood of the null line of each band. The identification of the electronic levels renders it possible to calculate the positions of certain other bands which should occur, and one of these (5S → 2P, ortho-helium family) has since been found by Weizel and Füchtbauer. They have also recorded a number of bands involving the first vibration state of the molecule. In the present paper it is intended to give details of three new bands which have the same final electronic state (2P) as the three which were first investigated, to discuss their relationships and interpretation, and to derive molecular constants for all six bands. Before proceeding with this it is desirable briefly to outline the changes in notation which are necessitated by the recent advances mentioned above. The most radical of these is the re-numbering of the lines according to Mecke’s suggestion that alternate members of all branches are missing. As this view is now generally accepted by workers in this field, no more than a brief summary need be given of the evidence on which it is based.