Product branching fractions for the reaction of O(3P) with ethene

Abstract

The product branching fractions for the reaction of atomic oxygen with ethene, O(³P) + C₂H₄→ CH₃ + HCO (1a), → H + CH₂CHO (1b), → H₂ + CH₂CO (1c), have been investigated at room temperature (295 ± 4 K) and pressures from 1 to 4 Torr (with N₂ or He buffer) by a laser photolysis–photoionization mass spectrometry method. From the yield of CH₃ radical, ϕ(CH₃), the branching fraction for (1a) was determined to be 0.53 ± 0.04 and no apparent pressure dependence was found from 1.5 to 4.0 Torr (N₂buffer). The ratio of the HCO yield to that of CH₃, ϕ(HCO)/ϕ(CH₃), was measured to be less than unity and increased as pressure increased (∼0.7 at 1 Torr and ∼0.9 at 4 Torr [He]) suggesting prompt dissociation of the hot HCO radical (to H + CO) formed by channel (1a) at low pressures. An interpretation which reduces the large discrepancy among branching fractions reported for low pressure region is presented. The existence of the molecular H₂-elimination channel (1c) was confirmed. The branching fraction for channel (1c) was determined to be 0.019 ± 0.001 by the yield of CH₂CO and was independent of pressure from 1.0 to 4.0 Torr (He buffer). As a side result, the yield of CH₃ radical from O(¹D) + C₂H₄ reaction was also determined.