On the shape of pressure-broadened absorption lines in the microwave region. I. Deviations from the Lorentzian line shape

Abstract

In microwave spectroscopy it is generally accepted that the line shape of the rotational absorption lines is Lorentzian. In 1972 Parsons et al. confirmed experimentally that this assumption was extremely accurate. A study is made of the influence of the absolute velocity distribution of absorbing molecules on the line shape of a microwave absorption line broadened by collisions only. It will be shown that the transition rate due to collisions depends on the absolute velocity of the absorbing molecule. As a consequence, the line shape is given by a super-position of Lorentzian line shapes, each of which corresponds to a class of absorbing molecules with a specific absolute velocity.