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 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.

Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer: 2. Determination of the mean composition of the upper troposphere and stratosphere

“…Rate values from Chadwick et al (1993) have been used for C 2 H 2 and extrapolated to the other hydrocarbon species. For methane, in addition to the collisional rates used by Martín-Torres et al (1998), where only collisions with N 2 , O 2 , where taken into account, we also used relaxation rates for CH 4 -H 2 from Hess and Moore (1976) for the V = 3 levels and Menard-Bourcin et al (2000, 2001, 2005 for the other states. For CO 2 , we used the widely accepted values from Nebel et al (1994).…”

Temperature dependence of rotational relaxation of methane in the 2ν3 vibrational state by self- and nitrogen-collisions and comparison with line broadening measurements

“…Particularly, non thermal emissions were observed from the atmospheres of giant planets: Saturn, Jupiter [1] and their satellites, especially Titan [2][3][4][5]. In previous works, rotational depopulation rates of methane were measured at room temperature [6,7] and at 193 K [8,9], a temperature range adapted to terrestrial applications. The purpose of the present work is to investigate the rotational relaxation of methane at lower temperatures more suitable for these atmospheres and to study this relaxation in CH 4 -N 2 mixtures, nitrogen being the major constituent of Titan.…”

Identification of hot band transitions of CH4 near 3000cm−1

“…The following decrease in absorption of the signal is due to rotational energy transfer depleting the pumped level. A detailed discussion of this process has been reported in previous articles dedicated to the study of rotational relaxation processes in CH 4 and CH 4 + N 2 , H 2 , or He mixtures [21][22][23][24]. With the pump tuned to the Q (3), R (0), Q (4), or Q (6) line, the same probe frequency leads to weaker DR signals with a slower decay.…”

“…As it has been discussed in our previous articles [21][22][23][24] dedicated to relaxation studies, since the time behavior of the DR signals is related to the time evolution of the transient population of the upper and lower levels of the probed transition, the shape of these signals is a signature of the transition.…”

“…Detailed descriptions of our DR experimental setup can be found in [21][22][23][24]. Methane molecules are excited into selected rovibrational levels of the 2m 3 (F 2 ) vibrational substate near 6000 cm À1 by the 7 ns output pulse of an optical parametric oscillator (OPO) pumped by a single mode pulsed Nd: YAG laser (Continuum).…”