Ionic Collision Processes in Water Vapor

Abstract

Some ionic collision processes in water vapor have been studied by the techniques of high‐pressure single‐source mass spectrometry. Rate coefficients for reactions resulting from collisions of H₂O⁺ with H₂O, and of OH⁺ with H₂O have been studied as a function of primary‐ion translational energy over the energy range of 0–8 eV. For purposes of calibration and comparison, the reaction leading to CH₅⁺ production in methane gas has also been studied over the same primary‐ion energy range. The rate coefficient for H₃O⁺ production from H₂O⁺ / H₂O collisions was found to drop from a value of 26 × 10⁻¹⁰ cm³ molecule⁻¹·sec⁻¹ for thermal ion reactions to a value of less than half of this for ions with a maximum energy of 4 eV. H₃O⁺ production from OH⁺ / H₂O collisions was found to proceed with a rate coefficient of 20 × 10⁻¹⁰ cm³ molecule⁻¹·sec⁻¹ for ions of thermal energy, but this coefficient had dropped to zero as the maximum ion energy was increased to 0.8 eV. Evidence is presented to support the contention that although H₃O⁺ production decreases as the OH⁺ energy increases, the rate of removal of OH⁺ continues with high cross section The results indicate that in the ion energy region above 0.8 eV, OH⁺ is removed exclusively by charge transfer to H₂O.