Kinetics and product study of the reaction of CH3 radicals with O(P-3) atoms using time resolved time-of-flight spectrometry

The kinetics and product distribution for the reaction of methyl radicals, CH3, with ground-state O(P-3) oxygen atoms have been investigated. This rea ction was studied with a newly constructed apparatus combining a tubular fl ow reactor and a time-of-flight mass spectrometer (TOFMS), using a hollow-c athode lamp for photoionization. The radicals are produced by an excimer la ser pulse (lambda = 193 nm) in the cophotolysis of acetone, CH3COCH3, or br omomethane, CH3Br, and sulfur dioxide, SO2, creating a homogeneous distribu tion of radicals along the axis of the flow reactor. A small fraction of th e reaction mixture is sampled through a pinhole in the wall. Subsequent ion ization and repeated extraction of ionized molecules into the TOFMS at a hi gh repetition rate (approximate to 20 kHz) allows the simultaneous observat ion of rapid changes in the concentration of multiple species in the flow r eactor. In addition to the dominant product, formaldehyde (CH2O), carbon mo noxide (CO) was detected as a product with a yield of 0.17 +/- 0.11. Analys is of the rate of disappearance of methyl radicals and appearance of formal dehyde for different O(P-3) concentrations resulted in an overall rate coef ficient for this reaction k = (1.7 +/- 0.3) x 10(-10) cm(3) molecule(-1) s( -1) at T = (299 +/- 2) K and P = 1 Torr (He).