The absorption spectra of polyatomic molecules containing methyl and ethyl radicals—III

Abstract
The study of the absorption spectra of simple polyatomic molecules has many objects. As for diatomic molecules, it may lead to a deeper understanding of the interpretation of the spectra of polyatomic molecules in terms of spectral theory. As a consequence it may also lead to chemical data such as the energies of linkage, interatomic distances, and the electronic configurations of the molecules. In addition, it may help to explain the mechanism and nature of the primary processes in photochemical decomposition. In passing from diatomic to polyatomic molecules, however, two additional matters become of importance: on the one hand, the nature and types of vibration (valency or deformation, symmetrical or antisymmetrical) induced by the electronic excitation are of interest, and, on the other, it is desirable to examine how far symmetry properties assumed on the basis of independently determined molecular structures are borne out in the structure of their spectra. The difficulties in these studies are not solely due to the often very complicated structure of polyatomic molecule spectra, but also frequently to the continuous nature of these spectra in the ultra-violet region. It is with this region of absorption that photochemical considerations are usually concerned, the frequencies associated with it corresponding to transitions between the lower electronic states of the molecules. The interpretation of the continua is at the present time far from clear. In some cases work at low pressures reveals the existence of bands which so merge together with increasing pressure as to give effectively continuous absorption, whilst in other cases where the continua are apparently genuine, the influence of pressure upon the long wave limit of absorption is considerable. Even the conclusions to be drawn from the diffuse nature of the bands with polyatomic molecules are usually ambiguous, since it is not certain whether true cases of "predissociation" are involved or whether the large moments of inertia lead to unresolved close packing of rotational lines.

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