Abstract
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 2ν3(F2) state near 6000 cm-1. By probing with a tunable laser diode the 3ν3−2ν3(F2) 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-1Torr-1 and (21.5 ± 3.0) μs-1Torr-1, respectively, for self- and CH4−N2 collisions. By probing other stretching transitions such as 2ν3(F2)−ν3, (ν3 + 2ν4)−2ν4, and (ν3 + ν4)−ν4 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. Furth...
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