Chemiluminescence of CH in the O +C2H2 Reaction : Rotational Relaxation and Quenching

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Abstract
Rotational distributions of CH (A 2Δ, v=0) were measured at pressures from 0.1 to 8.5 torr in the O+C2H2 reaction in the presence of large excess of N2, Ar, or He. The reaction N+NO→N2+O was used to produce O without O2. Under all conditions the distributions are found to be a superposition of two Boltzmann distributions, one at 1200°—1400°K characteristic of the process leading to the formation of excited CH (A 2Δ), and one which stays close to the temperature of the reactor. Assuming that the observed behavior of the rotational distributions is caused by collisions of excited CH with heat‐bath molecules and interpreting the fraction of molecules in this low‐temperature distribution as a measure of the extent of relaxation of the initial rotational distribution of excited CH, average relaxation rate constants are derived. With the collision partners mentioned above, 10 to 30 collisions are required for relaxation. A 14‐level phenomenological model including only transitions between neighboring rotational levels (K→K±1) fits the experimental distributions well at all pressures studied if the downward rate constants kK−1,K are approximately proportional to exp(−2BK/kT), where B=14.6 cm−1 is the rotational constant for CH(A 2Δ, v=0) and T=360°K. Other models are discussed. Rotational and vibrational distributions for CD(A 2Δ) are also described. Quenching of the CH(A→X) emission by added oxygen was studied quantitatively and the results are consistent with a mechanism in which not CH(A 2Δ) but a precursor to its formation is removed by O2. In the absence of O2 the lifetime of the precursor is determined primarily by the atomic‐oxygen concentration. When an electrical discharge through O2 is used as the source of oxygen atoms for reaction with acetylene, the rotational distribution of CH(A 2Δ) and its pressure dependence is quite different from that described above. When acetylene reacts with the products of a discharge through CO2 with excess CO2 as heat bath, the rotational distribution of CH(A 2Δ) has a Boltzmann form at all pressures, with only the rotational temperature depending on pressure and approaching the gas kinetic temperature at high pressure.