Physico-chemical studies of complex organic molecules. Part II.―Absorption spectra at low temperatures

The physical criterion which has proved most valuable in distinguishing between different complex molecules is the absorption spectrum, but the absorption spectra have nearly always been measured in solution at ordinary temperatures, and usually give broad structureless bands. These bands frequently appear similar for different molecules, and are obviously unsatisfactory for the purpose of recognizing a particular molecule and distinguishing it uniquely. The ideal data for spectroscopic recognition are lines which are peculiar to a given atom or molecule. At low temperatures the bands of many molecules develop a structure with a considerable amount of detail (Kronenberger, 1930; Conant and Crawford, 1930; Arnold and Kistiakowsky, 1932; Spedding and Bear, 1933; Robertson, Fox, and Martin, 1934). This is probably due to two main factors: (i) the ground state of the molecule is simplified at low temperatures by the elimination of all but the vibrational states of lowest energy; and (ii) the Stark effects of the molecular fields of neighbouring molecules is rendered more constant as the molecules become quiescent, consequently the blurring is reduced. Both effects tend to diminish the width of the absorption bands and to bring out details of structure which cannot be recognized at atmospheric temperature. The development of structure will not be a property of all molecules; those, whose upper state of the electronic transition corresponds to dissociation or to "predissociation," will have bands continuous under all condition. But for a very large number of molecules, it should be possible to obtain structure at low temperatures, and in all cases narrowing of the bands should occur.