Pressure Broadening in the Microwave and Infra-Red Regions

TLDR

The theory incorporates collision‑induced transitions among quantum states, but relies on a classical picture of molecular motion and requires inclusion of short‑range forces for accurate agreement with experiment. The authors develop a generalized theory of collision broadening capable of predicting line breadths in the microwave and infrared regions. They derive formulas for computing approximate line‑broadening collision cross‑sections from known intermolecular interactions and validate them against experiments on self‑broadening in the ammonia inversion spectrum and the vibrational band spectra of HCl and HCN. The theory accurately predicts self‑broadening in ammonia and HCl/HCN spectra, but fails to explain foreign‑gas broadening in the microwave region, indicating that Van der Waals interactions alone are insufficient.

Abstract

In this paper a generalized theory of collision broadening is developed, adequate for predicting line breadths in the microwave and infra-red regions. This theory differs from previous ones in taking into account transitions among quantum states caused by collisions, although it is limited to a classical picture of the relative motion of the colliding molecules as a whole. Formulas are derived for computing approximate line broadening collision cross-sections from known intermolecular interactions, and the results of the theory are successfully compared with experiments on self-broadening in the ammonia inversion spectrum and the vibrational band spectra of HCl and HCN. Some cases of foreign gas broadening in the microwave region are examined, but it is concluded that in general the Van der Waals interaction, commonly assumed to be the force important in causing foreign gas broadening, is not adequate to cause the observed broadenings. The more complicated types of forces which become important at short range would have to be taken into account to give good agreement with experiment in these cases.