Time-Resolved IR−IR Double Resonance Measurements in Methane Excited to 2ν₃(F₂)

Time-resolved double resonance (DR) measurements have been performed in neat methane in order to investigate the rovibrational energy transfer processes occurring in CH 4 upon inelastic collisions. The CH 4 molecules were excited to the 2ν 3 (F 2 ) vibrational state by an optical parametric oscillator pumped by a Nd:YAG laser and tuned around 1.66 μm, and the low power beam of a tunable diode laser emitting around 3.4 μm was used to probe diad−octad, pentad−tetradecad, and transitions with the tetradecad as the lower level. The transitions involving the tetradecad are not yet well-known, but our DR measurements have allowed, in numerous cases, the assignment of 2ν 3 (F 2 ) ← ν 3 transitions, their frequencies being measured with a precision of ±0.01 cm -1, which should be useful for the spectroscopists who are analyzing the 2ν 3 vibrational state. Several rate constants have been deduced from the time evolution of the DR signals. A rate constant of 20 ± 2 μs -1 Torr -1 was obtained for the rotational energy transfer within 2ν 3 (F 2 ) occurring between levels of the same nuclear spin modification. Following the rotational equilibration, the 2ν 3 (F 2 ) levels relax first with a rate constant of 6 ± 2 μs -1 Torr -1 corresponding to rovibrational energy transfer within the tetradecad, then with a rate constant of 1.7 ± 0.2 μs -1 Torr -1 corresponding to the deexcitation of the tetradecad due to near-resonant energy transfer coupling the tetradecad to lower polyads. Other rate constants concerning the relaxation of the pentad and the diad have also been determined.