Direct measurement of rotational and vibrational relaxation in methane overtone levels by time-resolved infrared double-resonance spectroscopy

Abstract

A time-resolved infrared double-resonance technique has been used to measure vibrationally and rotationally inelastic collision rates in ground and vibrational overtone levels of methane. A Raman-shifted Ti:sapphire laser is used to pump J=0 through 7 states in the 2ν3 and ν3+ν4 levels of 12CH4, and a tunable diode laser is used to probe the time-dependent level populations. Vibrational equilibration is observed among the octad, pentad, and dyad levels, with subsequent relaxation to the ground state. State-to-state rotational energy transfer rates are obtained in the ground and ν3+ν4 excited vibrational levels, and compared with theoretical predictions and with pressure-broadening measurements on the corresponding transitions. The probability of molecular reorientation in an inelastic collision is also inferred from the polarization dependence of the relaxation times. Parity-conserving and vibrational angular momentum propensity rules are inferred for the lower rotational levels of methane.

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.…”

“…, L), the combinatorial states are denoted by lν i + kν j , and the energies of different states are given in brackets in Kelvin degrees. For the CH 4 molecule, the rate constants of the processes CH 4 (A) → CH 4 (B) (k AB ), CH 4 (B) → CH 4 (C) (k BC ), and CH 4 (C) → CH 4 (G) (k CG ) were measured in [18] at T = 300 K. The measurements showed that k AB ≈ k BC k CG . The characteristic times of these processes at atmospheric pressure are τ AB = 2.6 nsec, τ BC = 2.2 nsec, and τ CG = 1.5 μsec.…”

“…Because of the large number of the types of vibrations and, correspondingly, combinatorial vibrational states, many states that belong to different modes are in almost perfect resonance (have identical energies). Therefore, vibrational relaxation in such molecules is considered as the energy exchange between different groups including vibrational states with similar values of energy [18]. Figure 2 shows the schemes of vibrational levels of CH 4 and CH 3 molecules with identified channels of energy exchange between the groups A, B, and C for the CH 4 molecule and the groups A, B, C, and D for the CH 3 molecule; the ground state is indicated by the letter G. The states belonging to the ν i mode (i = 1, .…”

“…Here W H2O 5,0 (T ) is the temperature dependence of the rate constant of V-T relaxation of the deformation mode of the H 2 O molecule in a collision with M = H 2 O; W CH4 19,0 (T = 300 K) is the rate constant of V-T relaxation of vibrational energy in the CH 4 molecule (k CG ), which was measured in [18].…”

State-resolved collisional energy transfer in highly excited NO2. I. Cross sections and propensities for J, K, and mJ changing collisions

“…In contrast, at low internal energies, detailed studies are much more common, leading to a consistent physical picture of inelastic processes in this regime. 8,[19][20][21] NO 2 has also been the subject of numerous non-state-resolved spectroscopy based energy transfer measurements at quite high internal energies for nearly 2 decades, [22][23][24][25][26] but in these experiments the resolution was not sufficient to separate rotational energy transfer from the overall relaxation of the molecule.…”