Nuclear distances from band spectra

The nuclear distance can therefore be found if we know the values of the Deslandres terms, which can be determined from the emitted rotational band spectra. We have already pointed out (p. 101) that the spectrum emitted by a rotator consists of a series of equidistant lines. In fact, as we have already repeatedly stated, in simply periodic motions there is a selection rule A = dz 1, so that the emitted frequency is found as the difference of two consecutive energy terms ... 

The aim of molecular spectrum analysis is to reduce the vibrations observed in the infra-red, visible and ultra-violet band spectrum as well as in the Raman spectrum to a definite model locating exactly the individual atomic centers of mass on the one hand, and specifying quantitatively the forces between the constituent atoms on the other. The former object is relatively easy to attain from data on inter-nuclear distances and valence angles, while the latter is a difficult problem as yet unsolved. In interpreting band spectra, we have assumed that, among all the atoms of a molecule, including even those not directly united, forces interact which depend only upon the distances separating the atoms. 

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