Investigations in the infra-red region of the spectrum. Part II.—The absorption spectrum of sulphur dioxide

It has been pointed out that if the precautions indicated in the previous paper are carefully observed, a high order of accuracy in infra-red measurements can be obtained with a prism spectrometer. Although the final word at present rests with the grating instrument on account of its higher resolving power, nevertheless a deal of useful work remains to be done with prisms. A band can be located (generally a necessary process for a prism spectrometer before the grating can be applied), the band centre and band type can often be determined, and the molecular constants can be approximately deduced. It is proposed in these papers to examine the spectra of gases and vapours in which the molecure has three different moments of inertia. The rotational fine structure within a vibrational band due to an asymmetrical rotator is very complex and individual lines may lie so close and the constituent series so overlap that absorption may be practically continuous. The prism spectrometer enables the first step in the unravelling of the tangle to be accomplished, in that the envelope of the individual lines gives under favourable circumstances the branches of the Bjerrum doublets, and hence enables the moments of inertia to be determined and frequently provides a knowledge of what may be called the resultant electrical structure of the molecule.