Effect of Low Temperatures on Infra-Red Spectra

Abstract

A MAJOR portion of infra-red spectroscopy to-day is directed toward the determination of characteristic vibrational frequencies of polyatomic molecules. In almost all cases the information is limited because the broad bands overlap. Cooling the sample to low temperatures offers the possibility that the natural width of each band is reduced sufficiently to resolve individual vibrational frequencies. The extent of the improvement has recently been the subject of a considerable amount of controversy. Walsh and Willis¹ and we² have pointed out that, in general, quantized rotation does not occur in condensed phases, so that one cannot agree with Avery, Morrison and Ellis³, who stated that narrowing would obey the very strong law, ∼ T^½, characteristic of rotational structure. However, we cannot agree with Walsh and Willis that narrowing can only occur through a change of state. On the contrary, on very general grounds^2,4, some narrowing would be expected from depopulation of the Debye acoustic waves, which are coupled with the molecular vibrations to give width to the bands.