THE Wilson-Sommerfeld principle when applied to the case of a rotating molecule, supposed rigid, leads to the expression mh/π (m integral, h Planck's constant) for the angular momentum of the molecule, and on this basis, with the help of several other assumptions, it has been found possible to account very satisfactorily for the main features of the structure of band-series. The theory is found to apply not only to ordinary bands in the region of the visible spectrum but also to absorption bands of very different appearance, such as those due to the halogen acids which occur in the infra-red. In the latter case, however, a discrepancy exists which has given rise to a good deal of discussion. The observed bands consist of a number of lines distributed, to a first approximation, according to the formula where m takes successive positive integral values and is a constant. The line of wave-number v0 (i.e. m = 0) is invariably absent.
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Biographical Memoirs of Fellows of the Royal Society (1970)