Collisional interference in the infrared absorption spectrum of HD

The small dipole moment of HD makes it an almost unique case for the study of the interference between collision‐induced and allowed transitions. Normally, such effects are overshadowed by strongly allowed absorption. Experimental and theoretical aspects of the problem are reviewed with emphasis on the temperature dependence of the interference and line shape parameters of the pure rotational far‐infrared spectrum. The observation are not fully explainable in terms of the currently available theory.