화학공학소재연구정보센터
Journal of Physical Chemistry A, Vol.104, No.23, 5444-5450, 2000
Time-resolved IR-IR double resonance measurements in methane excited to 2 nu(3)(F-2)
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 2 nu(3)(F-2) vibrational state by an optical parametric oscillator pumped by a Nd:YAG laser and tuned around 1.66 mu m, and the low power beam of a tunable diode laser emitting around 3.4 mu m 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 2 nu(3)(F-2) <-- nu(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 2 nu(3) vibrational state. Several rate constants have been deduced from the time evolution of the DR signals. A rate constant of 20 +/- 2 mu s(-1) Torr(-1) was obtained for the rotational energy transfer within 2 nu(3)(F-2) occurring between levels of the same nuclear spin modification. Following the rotational equilibration, the 2 nu(3)(F-2) levels relax first with a rate constant of 6 +/- 2 mu s(-1) Torr(-1) corresponding to rovibrational energy transfer within the tetradecad, then with a rate constant of 1.7 +/- 0.2 mu s(-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.