Ion–Molecule Reactions, He++O2 and He++N2, at Thermal Energies and Above

Abstract

The rate coefficients for the reactions of He⁺ ions with N₂ and O₂ have been measured in a drift tube–mass spectrometer from thermal energies (300°K) to ∼ 0.1 eV. In addition to reaction rate determinations by the usual product‐ion “arrival spectrum” method, a new technique involving “additional residence time” for the parent ion is described which is applicable even when parent and product ions have equal mobilities. The total rate coefficient (i.e., the sum of dissociative and the ordinary charge transfer) for He⁺+N₂ reaction was found to be (1.0 + 0.3, − 0.2) × 10⁻⁹ cm³/sec at 300°K, decreasing to ∼ 7 × 10⁻¹⁰ cm³/sec at 0.1 eV. The corresponding coefficient for the He⁺+O₂ reaction was measured to be (8.5 + 2.5, − 2.0) × 10⁻¹⁰ cm³/sec and to be independent of energy over the same range. The ratio of dissociative charge transfer (with N⁺ or O⁺, respectively, as reaction products) to ordinary charge transfer (with N₂⁺ or O₂⁺, respectively, as products) was found to be 0.55 / 0.45 for He⁺+N₂ and 0.8 / 0.2 for He⁺+O₂.