Merging-Beams Studies of Ba Collisions with Atmospheric Molecular Ions

Ion-molecule reactions between Ba and some atmospheric molecular ions have been studied using a merging-beams apparatus. The interaction energy $W$ (i.e., relative kinetic energy in the center-of-mass system) was varied from 0.1 to 10 or 20 eV. The processes investigated were Ba + ${\mathrm{O}}_2^{+}$ → Ba${\mathrm{O}}^{+}$ + O, Ba + ${\mathrm{N}}_2^{+}$ → Ba${\mathrm{N}}^{+}$ + N, and Ba + N${\mathrm{O}}^{+}$ → Ba${\mathrm{N}}^{+}$ + O and Ba + N${\mathrm{O}}^{+}$ → Ba${\mathrm{O}}^{+}$ + N. There is evidence that each process proceeds via a direct rather than a complex mechanism. In particular, in the center-of-mass system most of the product ions are scattered in the direction of the reactant Ba. Analysis of the kinetic energy of the products indicates that they are formed with considerable internal energy. The relative cross section for each of the reactions decreases as $W$ increases. At $W=0.1\mathrm{}$ eV the absolute cross section for each process is of the order of ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$. Studies were made of the dependence of the cross section on internal excitation of the reactants.