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Deviations of experimental millimetre-absorption line shapes from the usual Voigt profile are evidenced for two room-temperature rotational transitions J ¼ 20 19 (502 GHz) and J ¼ 24 23 (602 GHz) of nitrous oxide perturbed by nitrogen and oxygen. These deviations are shown to be due to collisional narrowing processes encompassed by more sophisticated speed-dependent Voigt or Galatry models. The deduced collision-regime line broadening coefficients are compared to their theoretical estimations by a semi-classical formalism. Even with a rough intermolecular potential composed of electrostatic and pair atom-atom interactions, the theoretical linewidths compare favourably with experimental results for both millimetre and infrared absorption.

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The model of exact trajectories involved in a semiclassical computation of ͑vib͒rotational linewidths and shifts is extended to the case of asymmetric top colliders. General expressions for the second-order contributions to the scattering matrix are given which define the pressure broadening and shift of the line. This theoretical approach is tested on the particular case of the infrared 7 band linewidths of C 2 H 4 broadened by N 2 which is frequently required for atmospheric applications. The computed linewidths compare favorably with available experimental data.