Reaction Window in the single-electron capture by Ammonia Dications.

Single-electron capture (SEC) by (NH3)2+ with rare gases (He, Ne, Ar, Kr, Xe) and benzene, (NH3)2++G->NH3++G+, has been investigated using a forward geometry double-focussing mass spectrometer. The SEC cross-section goes through a maximum within a reaction window centered at an exothermicity of approximately 4.5 eV. This corresponds to a target-projectile distance for the charge transfer of about 3.2 Angstöm. The average internal energy of NH3+-ions resulting from SEC has been obtained both via translational energy gain measurements and via an analysis of the daughter ion spectra. The agreement between both methods is satisfactory and shows that the lower the ionisation energy of the target, the more probable the excitation of NH3+ to the A2E state. With benzene (ionization energy=9.25 eV) as the target gas, however, the excess energy is converted partly to internal energy of C6H6+, leading most probably to the D2B2u, E2B1u and F2A1g states of C6H6+. A model based on the Landau-Zener non-adiabatic formalism, including one entrance channel (NH32++G) and two exit channels (NH3+ X2A"2+G+ and NH3+ A2E+G+) accounts qualitatively for the observed results in the case of exothermic single-electron capture. This model is not appropriate for slightly exothermic and endothermic charge transfer, i.e., in the present case, for the Ne and He target gases.