Branching ratios and incubation times in the thermal decomposition of methyl radicals: Experiments and theory

The thermal decomposition of methyl radicals proceeds cia two competitive channels: CH(3) + M --> CH(2 Pi) + H(2) + M (R1) CH(3) + M --> CH(2)((3)B) + H + M (R2) with an energy difference of about 13 kJ mol(-1) favoring the CH and H(2) channel (R1). In the low-pressure regime only the reaction channel with the lowest threshold should be open, while with increasing total pressure the channel with the higher threshold may compete. Collisional transfer of both energy and angular momentum controls this competition in the fall-off range. We investigated the thermal decomposition of methyl radicals behind reflected as well as behind incident shock waves at pressures between 100 and 4800 mbar and temperatures between 2000 and 4000 K. The CH(3) radicals were prepared by thermal decomposition of a few ppm of two different precursor molecules, azomethane and acetone, respectively, diluted in argon. The formation of H atoms during the decomposition was detected by atomic-resonance-absorption spectroscopy (ARAS) at 121.6 nm. We found pronounced incubation times in the H atom formation when the unimolecular decomposition of methyl radicals follows the very fast decomposition of the methyl precursor under identical shock wave conditions. We observed unexpectedly high H atom yields leading to branching ratios up to 70% for the channel (R2), depending on the experimental conditions. Both, the observed incubation times as well as the branching ratios in the methyl decomposition were analyzed in terms of master equation modeling using statistical unimolecular rate theories for the decomposition channels and a simple model for collisional energy transfer.