Temperature effects on the far-infrared absorption spectra of simple organic molecules

Abstract

The recent development of interferometers operating in the far-infrared frequency region, coupled with the use of modern high-speed digital computers for processing the interferometric data, has made this frequency region more readily available for investigation. It became apparent quite early that most, if not all, compounds exhibit absorptions in this region, confirming the speculations of earlier dielectrics studies. For polar molecules, the existence of these absorptions has been attributed to a variety of causes, several of which have in common a concept of a short-lived pseudo-lattice or cage of molecules surrounding a central trapped molecule which could absorb energy from an applied oscillating electromagnetic field in order to move within the potential energy well created by the cage members. In the present work, an attempt has been made to examine this concept by varying the temperature of the sample, with a view to altering the size and rigidity of the cage. The type of motion exhibited by the central molecule has also been considered. In general, both the intensity and frequency of maximum absorption increased smoothly with decreased temperature within the liquid phase, and within certain solid phases in which molecular rotational motion was allowed. On passing into non-rotator solid phases, however, the broad, featureless absorption curve of the liquid was replaced abruptly by one containing several sharp, narrow and intense absorption peaks. It has been suggested that the broad absorption bands of the liquid phase are composed of several overlapping bands due to different types of molecular motion. Further, each of the component bands was suggested to arise from the differences between the kinetic energy possessed by a central molecule, and the energy barrier created by the molecular cage. The kinetic energy of the central molecule was suggested to be governed by a continuous distribution. Similarly, the energy barrier might also be governed by a second continuous distribution.