Endoergic ion–molecule-collision processes of negative ions. I. Collision of I− on SO2

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Abstract
A new method has been developed for the investigation of endoergic ion–molecule-collision processes of negative ions. This method is, in effect, an extension of an earlier one developed by Refaey and Chupka used in connection with positive ions. An ion source with separate ionization and collision chambers was constructed and mounted on a Bendix time-of-flight mass spectrometer. I− ions produced in the ionization chamber by electron resonance capture of HI are pulsed to the desired energy where they enter a collision chamber containing a different gas. Cross sections for various endoergic ion–molecule-collision processes are measured as a function of the incident ion (I−) energy. Collision of I− on SO2 gives the secondary ions SO2−, IO−, O−, SO−, and S− and/or O2− in the energy range 2–25 eV (cm). Experimental thresholds were used to determine heats of formation as well as the electron affinities of various species. The electron affinities of SO2, IO, SO, and S were determined as 1.05, 2.3, 1.2, and 2.0 eV. The onset for the formation of O− indicates that it is accompanied by the neutral ISO. This onset leads to ΔHf(ISO) =−16 kcal mole−1. IS has been suspected as a product in two separate cases. A fairly good estimate of ΔHf(IS) can be set as 74 kcal mole−1.

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