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 prin cipal termolecular ion-molecule reactions that occur in the atmosphere of T itan, 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 we re studied in a nitrogen bath gas at room temperature. Only two of these io ns, 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 t he reactivity of ions in Titan's atmosphere that do not undergo rapid bimol ecular reactions with other species. The reactions of these so-called "term inal" ions (HCNH+, c-C3H3+, and H3O+) with the trace neutral constituents o f Titan (CH4, C2H2, and C2H4) were also examined using both helium and nitr ogen as the bath gas. The only reactions observed to occur were the termole cular reactions of HCNH+ and H3O+ with C2H2 and C2H4. A third set of reacti ons was studied. These were the reactions of N+ and Ni with the 4 species C H4, 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 t he similar reactions of N+ and N-2(+) with these reagents. Finally, a proje ction is made using a model temperature and pressure profile of Titan's atm osphere to appraise the relative importance of N+ vs N-3(+) reactions and N -2(+) vs N-4(+) reactions. (C) 2000 Academic Press.