Rovibrational Energy Transfer in Methane Excited to 2ν₃ in CH₄−N₂ Mixtures from Double-Resonance Measurements

The rovibrational energy transfer processes occurring in methane upon inelastic CH 4 −N 2 collisions have been investigated by using time-resolved double-resonance (DR) measurements. The CH 4 molecules were excited at about 1.66 μm into selected rotational levels of the 2ν 3 (F 2 ) state by an optical parametric oscillator pumped by a Nd:YAG laser. The low power beam of a tunable diode laser emitting around 3.4 μm was used to probe the following transitions: (ν 3 + ν 4 ) ← ν 4, 2ν 3 ← ν 3, (ν 3 + 2ν 4 ) ← 2ν 4, and 3ν 3 ← 2ν 3 transitions. Measurements were performed in CH 4 −N 2 gas mixtures with various molar fractions of methane. Rate constants were deduced from the time evolution of the transient populations of the probed levels. N 2 has been found to be an efficient collision partner to deplete the laser-excited level through rotational energy transfer, with a rate constant of (13.0 ± 1.5) μs -1 Torr -1 and through intermode transfer to equilibrate the populations among the states of the tetradecad, with a rate constant of (2.0 ± 0.6) μs -1 Torr -1 . These measurements have allowed us to determine the part played in the vibrational relaxation by intermode transfer processes, which occur between the strongly interacting states of the polyads upon CH 4 −CH 4 as well as CH 4 −N 2 collisions, and by near-resonant V−V energy transfer processes coupling the tetradecad to lower polyads, which occur only upon CH 4 −CH 4 collisions.