Time-resolved double resonance (DR) measurements have been performed in neat methane in order to investigate the rovibrational energy transfer processes occurring in CH4 upon inelastic collisions. The CH4 molecules were excited to the 2ν3(F2) 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 2ν3(F2) ← ν3 transitions, their frequencies being measured with a precision of ±0.01 cm-1, which should be useful for the spectroscopists who are analyzing the 2ν3 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 2ν3(F2) occurring between levels of the same nuclear spin modification. Following the rotational equilibration, the 2ν3(F2) 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.

A series of time-resolved IR-IR double-resonance experiments have been conducted where methane molecules are excited into a selected rovibrational level of the 2nu3(F2) vibrational substate of the tetradecad and where the time evolution of the population of the various energy levels is probed by a tunable continuous wave laser. The rotational relaxation and vibrational energy transfer processes occurring in methane upon inelastic CH4-H2 and CH4-He collisions have been investigated by this technique at room temperature and at 193 K. By probing transitions in which either the lower or the upper level is the laser-excited level, rotational depopulation rates in the 2nu3(F2) substate were measured. The rate constants for CH4-H2 collisions were found to be 17.7 +/- 2.0 and 18.9 +/- 2.0 micros(-1) Torr(-1) at 296 and 193 K, respectively, and for CH(4)-He collisions they are 12.1 +/- 1.5 and 16.0 +/- 2.0 micros(-1) Torr(-1) at the same temperatures. The vibrational relaxation was investigated by probing other stretching transitions such as 2nu3(F2) - nu3, nu3 + 2nu4 - 2nu4, and nu3 + nu4 - nu4. A kinetic model, taking into account the main collisional processes connecting energy levels up to 6000 cm(-1), that has been developed to describe the various relaxation pathways allowed us to calculate the temporal evolution of populations in these levels and to simulate double-resonance signals. The different rate coefficients of the vibrational relaxation processes involved in these mixtures were determined by fitting simulated signals to the observed signals corresponding to assigned transitions. For vibration to translation energy transfer processes, hydrogen is a much more efficient collision partner than helium, nitrogen, or methane itself at 193 K as well as at room temperature.

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.

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 ) r ν 4 , 2ν 3 r ν 3 , (ν 3 + 2ν 4 ) r 2ν 4 , and 3ν 3 r 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.