Vibrational and rotational collisional relaxation in CO2–Ar and CO2–He mixtures studied by stimulated Raman-infrared double resonance

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The collisional relaxation among vibrational levels of the Fermi dyad of CO2 mixed with Ar and He (10% CO2, 90% rare gas) has been studied at room temperature with a double resonance experiment. Stimulated Raman effect from the ground state achieved the pumping process with a Nd:YAG laser and a pulse amplified dye laser. After pumping the v1 or 2v2(Σ+g) level, a cw CO2 laser was used to probe either the depopulation rates of the pumped levels (vibrationally or rotationally resolved) or the energy transfer rates to neighboring states. The vibrational energy relaxation has been studied from experimental depopulation of v1 and population of 2v2 levels through a five-level model. A full set of vibrational rates has been determined and compared with other experimental and theoretical works when available. Our measurements could stimulate new accurate calculations. The rotational energy transfer among the J-resolved rotational levels of 2v2 has been observed for J=14 to 34 with a maximum difference between pumped and probed levels of ΔJ=20. The numerous experimental data have been modeled through a relaxation model based on the energy corrected sudden (ECS) approximation. The adjustable parameters of the ECS matrix have been directly determined by fitting the experimental time-dependent population curves. The diagonal elements of this matrix agree well with experimental linewidths. The resulting ECS relaxation matrix can be used to calculate line shapes in high-pressure coherent Raman spectra of v1 and 2v2 bands or infrared absorption spectra. In contrast, the inelastic linewidths of 2v2 deduced from depopulation data are in disagreement with available experimental linewidths, as already shown in numerous previous studies of gases and mixtures.