Rovibrational Relaxation of Methane in CH₄−N₂ Mixtures:  Time-Resolved IR−IR Double-Resonance Measurements at 193 K and Kinetic Modeling

Measurements have been conducted in methane and methane−nitrogen mixtures at 193 K by means of a time-resolved IR−IR double-resonance technique. Methane molecules were excited into selected rotational levels of the 2ν 3 (F 2 ) state near 6000 cm -1 . By probing with a tunable laser diode the 3ν 3 −2ν 3 (F 2 ) transitions in which the lower level is the laser-excited level, rotational depopulation rates were measured. They were found to be equal to (28.3 ± 3.0) μs -1 Torr -1 and (21.5 ± 3.0) μs -1 Torr -1, respectively, for self- and CH 4 −N 2 collisions. By probing other stretching transitions such as 2ν 3 (F 2 )−ν 3, (ν 3 + 2ν 4 )−2ν 4, and (ν 3 + ν 4 )−ν 4 transitions, various vibrational relaxation processes were investigated. A numerical kinetic model, taking into account many collisional processes connecting energy levels up to 6000 cm -1, has been developed to describe vibrational relaxation. This model allowed us to reproduce observed double-resonance signals and to determine rate coefficients for various relaxation processes. Furthermore, the good agreement between computed and observed signals is encouraging for using this model to predict the time evolution of populations of methane energy levels especially for pressure or mixing ratio values that cannot be realized in our experiments.