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Session 15 - Interstellar Medium I.
Display session, Monday, January 15
North Banquet Hall, Convention Center
Much of the understanding of conditions in interstellar gas clouds comes from observations of emission and absorption by ambient molecules. To interpret these observations with confidence requires the synthetic routes to the molecules to be understood. A considerable effort has been devoted to explaining the synthesis by gas phase ion-molecule reactions, but there remain uncertainties. Important amongst these are electron-ion dissociative recombination, ion-molecule proton transfer reactions and ion-atom reactions. The recent determination of the fractional abundance of H2O in Sgr. B2 prompted our determination of the complete product distribution for H3O+ recombination and of trends in the competitive loss by proton transfer. This has shown that little H2O is produced in H3O+ recombination. Proton transfer reactions from H3O+ to a series of species with larger proton affinities, is non-dissociatve and rapid. Since proton transfer to H2O from species with smaller proton affinities is similar, the two types of proton transfer cycle water between H2O and H3O+, with the abundance of H2O being continuously depleted by H3O+ recombination. Limited modelling involving H2O, H3O+ and OH, is giving more understanding of this chemistry. Hydrocarbon ion reactions are the backbone for the production of observed organic interstellar molecules. Many neutral hydrocarbons are not detectable and determination of their abundances relies on ion chemical models. Reactions of hydrocarbon ions with H2 are not all rapid and there are many bottlenecks in the hydrocarbon chemistry. In regions where significant atomic H exists, atom reactions can circumvent the bottlenecks. A series of CnHm+ (n and m are small integers) reactions with H-atoms have shown that two classes of reactions occur, association and H-atom abstraction from the ion.
