Absolute State-Selected Cross Sections for Ion-Molecule Reactions of Importance in Planetary Ionospheres

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Many aeronomic techniques depend on knowledge of the cross section for reactions involving singly ionized atomic oxygen. In this project, these cross sections will be measured using a triple-quadropole double octopole (TQDO) mass spectrometer. Significant technical advances have been made in the application of this technique for absolute, total, state-selected cross section measurements. In previous work conducted by the investigators, state-selected beams of ionized oxygen with high purities and intensities were prepared for scattering experiments by controlling the collision energies for dissociative charge transfer collisions of singly ionized helium, neon, and argon with molecular oxygen in a radio frequency (RF) ion guide gas cell, and by applying appropriate effective ion trapping potentials to the RF octopole ion guide. The method also makes use of a differential retarding potential method for improving the kinetic energy resolution of the ionized oxygen reactant ions that are produced by the reactions of molecular oxygen with helium, neon, and argon. Absolute total cross section measurements for the ion-molecule reactions of atomic oxygen with molecular nitrogen, oxygen, hydrogen, carbon dioxide, carbon monoxide, and water are made down to thermal energies using the simple electrostatic aperture of the TQDO apparatus, even though the reactant ions are formed originally with a broad kinetic energy distribution. The investigators will continue these cross section measurements and develop new experimental schemes for absolute cross section measurements of ion-atom reactions relevant to planetary ionospheres.***