Ion-molecule reactions are of critical importance in initiating ion processing in the atmosphere of Titan.   Specifically, the reactions of N+ and N2+ with CH4 are of central importance to the ion chemistry of Titan.  The N+ reactant may undergo both dissociative and non-dissociative charge transfer with CH4, as well as C-N bond formation to produce HCNH+, and each set of reaction products plays a unique role in the subsequent ion chemistry.  Facile insertion of N+ into C-H bonds of CH4 to form chemically activated ions that decay by ejection of hydrogen atoms or molecules leads to the formation of HCNH+, which may serve as proton donor to a number of species. Careful measurements of kinetic energy distributions in this system provide important insight into the spin-forbidden and spin-allowed pathways accessible in the formation of reaction products.  The N2+ reactant only undergoes dissociative charge transfer reactions with CH4.  These studies address the complementary and competitive roles that energy resonance and favorable Franck-Condon factors play in charge transfer, where ionization of CH4 results in extensive changes in geometry. We employ the Velocity Map Imaging method to the detection of reaction products of low energy ion-molecule collisions studied in crossed beams.  By crossing a mass-selected ion beam with a pulsed neutral molecular beam, delayed pulsed extraction of product ions from the interaction region allows us to determine energy and angular distributions of reaction products over a range of collision energies.  The experimental results, in concert with Gaussian G2 calculations, provide a detailed picture of reaction pathways, reactive intermediate lifetimes, and energy disposal in ionic reactions important in astrochemistry.

Imaging ion-molecule reactions: Applications in astrochemistry

246th National Meeting (2013)

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