Pressure Broadening in the Infra-Red and Optical Collision Diameters

Abstract

The pressure broadening effects of a number of foreign gases on the infra-red absorption of methane at 7.65μ have been determined. Also the pressure broadening effects of a number of foreign gases on the absorption of carbon dioxide at 4.3μ and 14.8μ have been investigated. The data have been analyzed to obtain optical collision diameters for the various gases and the absorbers. By optical collision is meant any collision which is effective in interrupting the processes of radiation absorption. The data are discussed in terms of the Lorentz theory of pressure broadening and are shown to confirm it. From the results obtained, it is clear that the pressure broadening effects of certain gases on one absorber cannot be reliably extrapolated to predict the effects on another absorber. Also the effects at one wave-length are not in general the same at another wave-length for the same absorber. The optical collision diameter effective for a foreign gas and an absorbing gas at a particular wave-length is a specific function of the two gases and the wave-length. Empirical results on the shape of the curves obtained are presented. The deviation from Beer's law of absorption evidenced by isobutylene at 11.23μ is shown to be not due to pressure broadening.