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

Abstract

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 wit...