| Summary: | Absolute rate constants for the removal of CH2 (ã1A1) by Ar, NO, H2, and CH2CO have been measured over the temperature range 295-859 K by the method of long-path kinetic absorption spectroscopy. For quenching to the ground X̃3B1 state by Ar, the rate constant rises by a factor of 3 over this range from its room-temperature value. Both the absolute values and their increase up to ∼500 K can be explained by a mixed-state model of quenching, in which fractional populations of singlet states perturbed by nearby triplet levels and the relaxation of the corresponding triplet levels contribute to the measured removal rate. Above 500 K, there is a significant deviation between experiment and calculation, suggesting that additional high-energy perturbed singlet levels are affecting the decay process. In contrast, removal by H2 and NO, both considered to be predominantly by reaction, shows no temperature variation over this range, while removal by ketene shows a negative temperature dependence. The implications of the results to singlet methylene quenching and reaction processes at combustion temperatures are discussed. © 1992 American Chemical Society.
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