Crossed beam study of He⁺-O₂ charge transfer reactions in the collision energy range 0.5–200 eV

Abstract

Energy spectra and angular distributions of the O⁺ and O ⁺₂ product ions resulting from the He⁺-O₂ charge transfer reaction have been measured in the collision energy range 0.5–200 eV using the crossed-beam method. The O ⁺₂ ions represent only a minor fraction of the reaction products (0.2–0.6% over the energy range measured). In the dissociative charge transfer reaction, four main processes are identified leading to O+O⁺ reaction products in different electronic states. Two different mechanisms can be distinguished, each being responsible for two of the observed processes:(i) a long-distance energy-resonant charge transfer process involving thec ⁴∑ ⁻ _u (v′=0) state of O ⁺₂ and(ii) a slightly exothermic charge transfer process via the (III)²∏ _u state of O ⁺₂ (with the exothermicity depending on the collision energy). Angle-integrated branching ratios and partial cross sections (in absolute units) have been determined. The branching ratios of the individual processes show a pronounced dependence on the collision energy. At low energies, the O⁺ product ions are preferentially formed in the² P ⁰ and² D ⁰ excited states. The angular distributions of the O⁺ product ions show an anisotropic behaviour indicating an orientation-dependent charge transfer probability in the He⁺ −O₂ reaction.