Vibrational Energy Transfer in Methane Excited to 2ν₃ in CH₄−N₂/O₂ Mixtures from Laser-Induced Fluorescence Measurements

Vibrational energy transfer processes in CH 4 −N 2 /O 2 mixtures have been investigated by using the laser-induced fluorescence method. Methane molecules were excited to the 2ν 3 vibrational state by an optical parametric oscillator pumped by a Nd:YAG laser, and hot fluorescence from combination and overtone states was monitored for both infrared active modes: ν 3 near 3.3 μm, and ν 4 near 7.5 μm. The ν 3 fluorescence exhibits a decay fitted by two exponential functions of time, while the ν 4 fluorescence shows an increase followed by a decay. The time dependence of the observed fluorescence signals was used to derive rate constants which were found to vary linearly as a function of the molar fraction of CH 4 in the gas mixtures. A numerical kinetic model, in which the populations of vibrational states with given numbers of stretching and bending excitation quanta are assumed to be in instantaneous equilibrium, has allowed to reproduce the time evolution of fluorescence intensities and to determine rate coefficients for intermode and V−V transfer processes occurring upon CH 4 −CH 4 and CH 4 −N 2 /O 2 collisions.