Ion cyclotron resonance studies of endothermic reactions of UF6- generated by surface ionization

Abstract

Ion cyclotron resonance spectroscopy is used to investigate endothermic reactions of UF6-. At temperatures below those required for electron emission, UF6- is formed by negative surface ionization on a directly heated rhenium filament. The reactant ion is accelerated to a known energy and traverses the source region of the ICR cell in the direction of the magnetic field. Slow products from reactive and nonreactive encounters are confined in the trapping well and detected in the usual manner. The formation of UF7-, UF6-, UF5-, and UF3- from the reaction of UF6- with UF6 is studied over the energy range 0–40 eV in the center of mass. The cross section for UF7- formation exhibits a threshold at a relative kinetic energy of 2.7 eV. Reaction of UF6- with BF3 leads to formation of BF4- with a threshold of 1.6 eV in the center of mass. From these thresholds, E.A.(UF6) =4.9±0.5 eV is deduced and the bond dissociation energies D (UF5–F−) =108±6 kcal/mole and D (UF6–F−) =46±10 kcal/mole are calculated. The endothermic reaction of UF6- with SF6 to produce SF6- is also investigated, the cross section for the process exhibiting a gradual onset at ~7.5 eV, considerably above the thermodynamic threshold. The experimental techniques for using ion cyclotron resonance spectroscopy to study endothermic reactions of positive and negative ions formed by surface ionization are considered in detail.

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