LOW-TEMPERATURE ION TRAP STUDIES OF N+(3Pja) + H2(j) → NH+ + H

Using a low-temperature 22-pole ion trap apparatus, detailed measurements for the title reaction have been performed between 10 K and 100 K in order to get some state specific information about this fundamental hydrogen abstraction process. The relative population of the two lowest H-2 rotational states, j = 0 and 1, has been varied systematically. NH+ formation is nearly thermo-neutral; however, to date, the energetics are not known with the accuracy required for low-temperature astrochemistry. Additional complications arise from the fact that, so far, there is no reliable theoretical or experimental information on how the reactivity of the N+ ion depends on its fine-structure (FS) state P-3(ja). Since in the present trapping experiment, thermalization of the initially hot FS population competes with hydrogen abstraction, the evaluation of the decay of N+ ions over long storage times and at various He and H-2 gas densities provides information on these processes. First assuming strict adiabatic behavior, a set of state specific rate coefficients is derived from the measured thermal rate coefficients. In addition, by recording the disappearance of the N+ ions over several orders of magnitude, information on nonadiabatic transitions is extracted including FS-changing collisions.