Saturation of infrared absorption in gaseous molecular systems

Saturable absorbers for use in the infrared, consisting of molecular gases, are considered. A basic objective of the study is the understanding and prediction of saturation behavior. A model for the absorber is developed. Features considered include 1) strong interaction between the absorbing pair of vibrational-rotational levels and numerous neighboring levels; 2) spatial diffusion of molecules; 3) temperature gradients; 4) distortion of optical beams by the saturable absorber; 5) inhomogeneous broadening of absorbing transitions; 6) effects of geometry of the absorption cell. The model is intended to be quite general in its applicability, but is particularly considered in relation to absorption of CO2laser radiation by sulfur hexafluoride. The equations arising from the absorber model are complex, but under suitable assumptions can be simplified. Experiments have been performed to determine the values of the relevant parameters for SF6absorbers. These values are inserted into the general absorber model and the resulting equations solved by digital computation. The theoretical saturation behavior thus obtained is in reasonably good agreement with saturation behavior observed in experiments with SF6.