Infra-red investigations of molecular structure.—Part IV. The overtone of nitric oxide

The study of the molecule of nitric oxide described in Part II of this series could not be considered complete until the overtone had been found or proved not to exist. It seemed desirable, therefore, to settle this question before the analysis of nitrous oxide and other triatomic molecules was begun. The agreement of most of the results of Part II with the observations of Rassetti showed that the frequency of the centre of the fundamental was different from the ω0 derived from electronic band spectra in the way that was anticipated by Birge some years ago, and shown to apply quantitatively for carbon monoxide in Part III of this series. That is, the centre of theQ branch of the nitric oxide fundamental corresponds to ω0 (1-x ) in the usual way for an anharmonic oscillator. With this in mind, it was quite incredible that there should be no overtone 2ω0 (1-2x ); and the missing of it in the earlier work was explained by the strong water band in the neighbourhood of 2.6μ where the overtone was expected to occur.