Short time behavior of the dipole autocorrelation function and molecular gases absorption spectrum

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Abstract
A quasistatic theory of the dipole autocorrelation function for isolated molecular spectral lines has been developed and applied to linear molecules perturbed by atoms and molecules. The matrix elements of the evolution operator associated with the molecular dipole moment are treated to all orders in the interaction without any cutoff procedure. An analytical expression of the intensity distribution in terms of the intermolecular potential has been obtained which is easily tractable for involved molecular systems. Numerical tests of this theory have been performed for rotation–vibration lines of HCl–Ar, HCl–Xe, and CO2–Ar gas mixtures by comparing the present results with that of a unified theory of the line shape for molecule–atom couples, valid from resonances to the far wings and including all orders in the interaction. These tests justify the present approach and show a considerable reduction of the computation time required (by more than two orders of magnitude), allowing now the study of the polyatomic preturbers case. Application to infrared absorption lines of HCl self‐perturbed and perturbed by CO2 leads to a good agreement with the experimentally determined super‐Lorentzian profile.