Rovibrational Energy Transfer in Methane Excited to 2ν₃ in CH₄−N₂ Mixtures from Double-Resonance Measurements

Abstract

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.

Initiation of combustion by laser-induced excitation of molecular vibrations of reactants

“…The extended model includes 311 reversible reactions with the following species: For thermally nonequilibrium conditions, where the distribution of vibrational energy in reactive molecules is specified by vibrational temperatures T ξ , the set of chemical reactions should be supplemented by the channels of vibrational-energy intermode (VV ) exchange and the pathways of vibrationaltranslational (VT) relaxation. Within the framework of the mode-resolved model adopted in the present work, we considered the VV exchange between the symmetric, deformation, and asymmetric (ν 1 , ν 2 , and ν 3 ) modes of H 2 O molecules, the modes of molecules of H 2 (ν 4 ), O 2 (ν 5 ), OH(ν 6 ), N 2 (ν 7 ), NO(ν 8 ), and CO(ν 21 ), the symmetric, deformation, and asymmetric modes of molecules of NO 2 (ν 9 , ν 10 , ν 11 ), HO 2 (ν 12 , ν 13 , ν 14 ), O 3 (ν 15 , ν 16 , ν 17 ), and CO 2 (ν 18 , ν 19 , ν 20 ), and the VT relaxation processes for the modes ν 2 , ν 4 , ν 5 , ν 6 , ν 7 , ν 8 , ν 10 , ν 14 , ν 16 , ν 19 , and ν 21 . It is known that vibrational relaxation in hydrocarbon molecules proceeds much faster than the relaxation in diatomic and triatomic molecules [19].…”

“…Within the framework of the mode-resolved model adopted in the present work, we considered the VV exchange between the symmetric, deformation, and asymmetric (ν 1 , ν 2 , and ν 3 ) modes of H 2 O molecules, the modes of molecules of H 2 (ν 4 ), O 2 (ν 5 ), OH(ν 6 ), N 2 (ν 7 ), NO(ν 8 ), and CO(ν 21 ), the symmetric, deformation, and asymmetric modes of molecules of NO 2 (ν 9 , ν 10 , ν 11 ), HO 2 (ν 12 , ν 13 , ν 14 ), O 3 (ν 15 , ν 16 , ν 17 ), and CO 2 (ν 18 , ν 19 , ν 20 ), and the VT relaxation processes for the modes ν 2 , ν 4 , ν 5 , ν 6 , ν 7 , ν 8 , ν 10 , ν 14 , ν 16 , ν 19 , and ν 21 . It is known that vibrational relaxation in hydrocarbon molecules proceeds much faster than the relaxation in diatomic and triatomic molecules [19]. According to these data, the vibrations of hydrocarbon molecules were assumed to be in thermodynamic equilibrium with translational degrees of freedom.…”

Near-infrared laser photoacoustic detection of methane: the impact of molecular relaxation

“…A complete description of the spectroscopic properties of CH 4 is given in Ref. [15] and the rovibrational energy transfer processes in CH 4 -N 2 /O 2 mixtures are also discussed in a series a papers [15,16,24]. We just recall here the main spectroscopic characteristics of CH 4 molecules that are essential for the understanding of our experimental results.…”

Initiation of combustion of a CH4-O2 mixture in a supersonic flow with excitation of O2 molecules by an electric discharge

“…It was believed until recently that vibrational relaxation in hydrocarbon molecules proceeds rather rapidly, and the translational, rotational, and vibrational degrees of freedom of these molecules in combustion of hydrocarbon fuels (even in the presence of shock waves or in the detonation mode of combustion) could be assumed with good accuracy to be thermodynamically equilibrium. The measurements of the times of vibrational relaxation in a CH 4 molecule [18,19], however, show that this assumption can prove to be invalid. Therefore, a finite time of vibrational-translational relaxation in a CH 4 molecule was taken into account in constructing the thermally nonequilibrium model of chemical kinetics.…”

“…A typical decrease in gas temperature in the zone adjacent to the shock-wave front should also be noted; this decrease is more pronounced in the case of the thermal action of the discharge. Figure 4 shows the behavior of translational (T ) and vibrational (T ξ ) temperatures of the basic compo- 5 , and T 6 ), H 2 (T 7 ), OH (T 8 ), CO (T 15 ), and CH 4 (T 19 )] behind the shock-wave front (M 0 = 8 and β = 25 • ) for E s = 3 · 10 −2 J/cm 3 and p 0 = 10 4 Pa for the cases where the discharge energy is spent both on activation of O 2 molecules and on heating of the mixture. It is seen that the decrease in T behind the shock-wave front in the case of simple heating of the gas is caused by excitation of molecular vibrations of CH 4 and O 2 .…”