Kinetics of dissociative recombination of H3O+ ions with free electrons in premixed flames

Abstract

Mass-spectrometric measurements have been made of the axial concentrations of the positive ions along premixed, flat flames of H₂, O₂ and N₂, either with or without trace amounts of acetylene added to the unburned gases. In each case the principal positive ion is H₃O⁺, whereas the sole negatively charged species is the free electron. The main reaction governing the concentration of H₃O⁺ in the burned gases of such a flame without acetylene added is: H₃O⁺+ e^–⇌ H + H + OH (1) However, with acetylene in the burner supplies, chemi-ionisation in a flame's reaction zone produces amounts of H₃O⁺ far in excess of those for equilibrium. Thus H₃O⁺ disappears via the forward step of (1) in the burned gases, enabling its rate constant to be measured between 1820 and 2400 K. This ion–electron recombination coefficient was found to vary with temperature, T, according to: k₁=(3.6 ± 0.5)T^{–2.1 ± 0.7} ion^–1 ml s^–1. The magnitude of k₁ at 2000 K is (4.2 ± 0.6)× 10^–7 ion^–1 ml s^–1; theoretical considerations indicate that k₁ is proportional to T^–1.5.