Termolecular ion-molecule reactions in Titan's atmosphere, I - Principal ions with principal neutrals

The FA-SIFT experiment at Canterbury was used to determine some of the principal termolecular ion-molecule reactions that occur in the atmosphere of Titan, Saturn's satellite. The experiments were performed using both a pure nitrogen bath gas and a pure helium bath gas. The reactions of the ions N+, N-2(+), N-3(+), N-4(+), HCNH+, c-C3H3+ and H3O+ with molecular nitreogn were studied in a nitrogen bath gas at room temperature. Only two of these ions, N+ and N-2(+), were observed to react with N-2, forming N-3(+) and N-4(+), respectively. A second set of experiments was performed that measured the reactivity of ions in Titan's atmosphere that do not undergo rapid bimolecular reactions with other species. The reactions of these so-called "terminal" ions (HCNH+, c-C3H3+, and H3O+) with the trace neutral constituents of Titan (CH4, C2H2, and C2H4) were also examined using both helium and nitrogen as the bath gas. The only reactions observed to occur were the termolecular reactions of HCNH+ and H3O+ with C2H2 and C2H4. A third set of reactions was studied. These were the reactions of N+ and Ni with the 4 species CH4, C2H2, C2H4, HCN, and C4H8. All of the reactions in this third set have bimolecular ion-molecule kinetics, and they are compared in the text with the similar reactions of N+ and N-2(+) with these reagents. Finally, a projection is made using a model temperature and pressure profile of Titan's atmosphere to appraise the relative importance of N+ vs N-3(+) reactions and N-2(+) vs N-4(+) reactions. (C) 2000 Academic Press.