There have been few attempts at the resolution of the vibration-rotation bands of a diatomic molecule. In 1919 Imes was successful with the bands of three of the hydrogen halides, work which was later extended by Colby and Meyer; Czerny proved the existence of a doublet due to HI, but the weakness of the absorption prevented more detailed study; E. F. Lowry in 1924 failed to analyse the structureless doublets of carbon monoxide, although his apparatus was similar to that used by Imes. It does not seem possible that the fine-structure would reveal itself if a lower pressure of the gas were used (E. F. Lowry worked at one atmosphere pressure). The molecule of CO, like those of the hydrogen halides, has a permanent electric moment, and its bands must be similar in kind. Apart from HF, HCI, HBr, HI and CO, NO is the only other diatomic molecule with a permanent electric moment, and its choice as the subject of this research was natural. It is more definitely homopolar than the hydrogen halides, although the distinction is almost certainly one of degree; there was the interest of establishing the self-evident proposition that there is no fundamental difference in the bands of No and the bands HCI. There was also the advantage of knowledge of the electronic band spectrum of the molecule acquired by Guillery, Jenkins, Barton and Mulliken, and summarised by Mulliken. The thoroughness of this work makes NO one of the best-known of molecules to the spectroscopist. It has been mentioned in the introduction to Part I that throughout this series of papers there will be maintained the deal of correlation between infra-red and electronic band spectra. Accordingly, it became our aim to compare the constants of the molecule in the normal state as derived from electronic band spectra and as obtained from the direct measurements of the infra-red. The unexcited electronic state of the molecule measurements of the infra-red. The unexcited electronic state of the molecule is, of course, the only one with which infra-red observations are concerned.
Infra-red investigations of molecular structure. Part II.—The molecule of nitric oxide