Kinetics and Product Study of the Reaction of CH3 Radicals with O(3P) Atoms Using Time Resolved Time-of-Flight Spectrometry

Abstract

The kinetics and product distribution for the reaction of methyl radicals, CH3, with ground-state O(3P) oxygen atoms have been investigated. This reaction was studied with a newly constructed apparatus combining a tubular flow reactor and a time-of-flight mass spectrometer (TOFMS), using a hollow-cathode lamp for photoionization. The radicals are produced by an excimer laser pulse (λ = 193 nm) in the cophotolysis of acetone, CH3COCH3, or bromomethane, CH3Br, and sulfur dioxide, SO2, creating a homogeneous distribution of radicals along the axis of the flow reactor. A small fraction of the reaction mixture is sampled through a pinhole in the wall. Subsequent ionization and repeated extraction of ionized molecules into the TOFMS at a high repetition rate (≈20 kHz) allows the simultaneous observation of rapid changes in the concentration of multiple species in the flow reactor. In addition to the dominant product, formaldehyde (CH2O), carbon monoxide (CO) was detected as a product with a yield of 0.17 ± 0.11. Analysis of the rate of disappearance of methyl radicals and appearance of formaldehyde for different O(3P) concentrations resulted in an overall rate coefficient for this reaction k = (1.7 ± 0.3) × 10-10 cm3 molecule-1 s-1 at T = (299 ± 2) K and P = 1 Torr (He).