Cross sections for gas phase ion—ion recombination in H3O++ X–→ HX + H2O for X = Cl, Br or I

Abstract

Mass spectrometric measurements have been made of the concentrations of ions present in flames of H₂, O₂ and N₂ with trace quantities of C₂H₂ added, as well as one of the halogens Cl, Br or I. The principal charged species found were H₃O⁺, e^– and X^–, where X is a halogen atom. The hydronium ion is produced in the reaction zone of a flame in amounts greater than those for equilibrium at the temperature of the burnt gases. Recombination of charged species thus occurs and proceeds by two reactions; H₃O⁺+ X^–1→ H₂O + HX (I), H₃O⁺+ e^–1→ H + H + OH. (II) From a study of the rate of decay of H₃O⁺ for varying amounts of one of the halogens present, it proved possible to measure the ratio of the rate coefficients (k₁/k₂) of reactions (I) and (II). For each halogen k₁/k₂ was constant over the temperature range 1810–2570 K and was 0.10 ± 0.03 for Cl, 0.07 ± 0.02 for Br and 0.03 ± 0.02 for I. The resulting values of k₁ are also independent of temperature and are 4.1 ± 1.6 × 10^–8, 2.7 ± 1.0 × 10^–8 and 1.2 ± 0.7 × 10^–8 ion^–1 cm³ s^–1 for Cl, Br and I, respectively. The corresponding cross-sections (πσ²) are 2.1, 1.5 and 0.7 × 10^–17 m². The heats of reaction (I) are large and the products appear to be H₂O and HX with the latter either electronically excited or completely dissociated, but not in its ground electronic state.