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Probing the potential energy surfaces of elementary neutral reactions using dissociative charge exchange


ʹThe dissociation dynamics of H₃, H₃O and CH₅ and their isotopologs were studied by neutralizing their analogous cation by charge exchange. These cations are important species in the interstellar medium, while their neutral analogs represent fundamental elementary neutral abstraction/exchange reactions Hʹ+H₂[arrow to the right]H+ HʹH, OH+H₂[arrow to the right]H₂O+H, and Hʹ+CH₄[arrow to the right]H+CH₃Hʹ. This research represents a unique probe of the potential energy surfaces of these species in the region of the cation geometry. Energetically excited neutrals were generated in the gas phase following charge exchange of fast beam of cations with a slow moving electron donor, Cesium. The neutrals are unstable with respect to their lowest dissociation channel and the center-of-mass kinetic energy release of the fragments was measured using a time- and position sensitive detection techniques. The H₃O and CH₅ systems were also studied theoretically using quasiclassical trajectory calculations with an ab initio potential energy surface of the species. The study on H₃ and D₃ revealed that in the two-body dissociation the H₂/D₂ fragment is formed vibrationally excited. In comparing the two isotopologs, it was observed that the predissociation of certain Rydberg states were suppressed in D₃, favoring radiative decay to the ground state. Measurement of the branching fractions revealed that the three-body channel is increasingly dominant when the principal quantum number of the Rydberg state increases. The dissociation dynamics of H₃O revealed that only H₂O+H products are observed. The H₂O is formed vibrationally excited with a vibrational inversion. Differences in the magnitude of the vibrational inversion were also observed depending on the identity of the eliminated atom (H or D). This research may have immediate impact on the understanding of the chemistry of comets. The final system studied is that of CH₅+С and a select number of its isotopologs. Only dissociations into CH₄+H and CH₃+H₂ product channels were observed, with the H atom elimination channel dominating. By comparing the experimental and calculated branching fractions, it was shown that the CH₅+С is a fluxional molecule. The mixed isotopologs of both CH₅ and H₃O showed dissociations ejecting an H or D atom were not statistical and in each case H atom elimination was favored

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