Rotational and Vibrational Relaxation of Methane Excited to 2ν₃ in CH₄/H₂ and CH₄/He Mixtures at 296 and 193 K from Double-Resonance Measurements

Abstract

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.

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

“…The experimental setup is very similar to that described in previous papers [8,9] except for the double resonance cell. The double resonance cell containing the gas mixture through which the pumping and probing beams propagate has been modified, as illustrated in Fig.…”

“…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.…”

“…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).…”

“…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.…”