We calculate the spectral line shape of an isolated line from first principles, assuming that the translational motion is not statistically correlated with the evolution of the optical coherence, i.e., with the broadening. We use the known, realistic potentials for the influence of collisions on the translational motion and on the internal motion. We show that the calculated profiles do not agree, particularly at low pressures, with very precise experimental profiles of the P(2) line of CO in a bath of Ar. We establish that the source of the disagreement lies in the assumption of uncorrelated effects of collisions on the translational motion and the optical coherence associated with the internal degrees of freedom.

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Using both a difference frequency spectrometer and a Fourier transform spectrometer, we have measured transitions in the 12 (2)0<--01 (1)0 band of carbon dioxide at room temperature and pressures up to 19 atm. The low-pressure spectra were analyzed using a variety of standard spectral profiles, all with an asymmetric component to account for weak line mixing. For this band, we have been able to retrieve experimental line strengths and the broadening and weak mixing parameters. In this paper we also compare the suitability of the energy-corrected sudden model to predict mixing in the two previously measured Q branches 20 (0)0<--01 (1)0, the 11 (1)0<--00 (0)0, and the present Q branch of pure CO(2), all at room temperature.

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