Isotopic Studies of the Proton–Hydrogen Molecule Reaction

The atomic‐ion–molecule systems of protons or deuterons and isotopic hydrogen molecules have been the subject of an experimental study designed to provide data to be compared with recently reported classical trajectory calculations on an ab initio potential energy surface. Absolute cross sections (accurate to within an estimated error of ± 20%) for the following atomic‐ion exchange reactions $${\mathrm{D}}^{+}+{\mathrm{H}}_2\to \mathrm{HD}+{\mathrm{H}}^{+},$$ $${\mathrm{H}}^{+}+{\mathrm{D}}_2\to \mathrm{HD}+{\mathrm{D}}^{+},$$ $${\mathrm{D}}^{+}+\mathrm{HD}\to {\mathrm{D}}_2+{\mathrm{H}}^{+}$$ have been investigated in a tandem mass spectrometer system, with carefully calibrated geometry, as a function of collision energy. Endothermic charge‐transfer processes $${\mathrm{D}}^{+}+{\mathrm{H}}_2\to {\mathrm{HD}}^{+}+\mathrm{H},$$ $${\mathrm{D}}^{+}+{\mathrm{D}}_2\to {\mathrm{D}}_2^{+}+\mathrm{D},$$ $${\mathrm{D}}^{+}+\mathrm{HD}\to {\mathrm{HD}}^{+}+\mathrm{D}$$ $$\to {\mathrm{D}}_2^{+}+\mathrm{H},$$ $${\mathrm{H}}^{+}+{\mathrm{H}}_2\to {\mathrm{H}}_2^{+}+\mathrm{H},$$ $${\mathrm{H}}^{+}+{\mathrm{D}}_2\to {\mathrm{HD}}^{+}+\mathrm{D}$$ $$\to {\mathrm{D}}_2^{+}+\mathrm{H}$$ have been observed, and their cross sections were determined as a function of collision energy. Energy distributions of the ionic species participating in these reactions were measured by application of appropriate retarding fields. The experimental results for atomic‐ion exchange reactions [(1)–(3)] are not easily compared with theoretical cross sections because the latter were calculated above energy thresholds for endothermic charge‐transfer reactions [Reactions (4)–(10). These processes were not considered in the theoretical treatment, though our results show that, where energetically possible, they indeed dominate the reactive collisions. The extent of the observed endothermic charge transfer was unexpected, but can be rationalized by postulating that very efficient translational to internal energy conversion occurs, followed by radiationless transition to a weakly bound or repulsive excited electronic state of H₃⁺. This ${}^1\mathrm{E\prime }$ state has been identified by a simple Hückel calculation. Comparisons of energy distributions of product and reactant ions superficially indicate “complex formation” at relatively low collision energies and “direct reaction” at higher energies for atomic‐ion exchange reactions [(1)–(3)]. The efficient transfer of kinetic to internal energy in the endothermic charge‐transfer processes suggests complex formation mechanisms. This idea is further supported by isotopic abundance data. Although the energy distributions of atomic‐ion exchange products above the endothermic charge‐transfer threshold are consistent with a direct mechanism, it should be noted that the relative yield of atomic‐ion products compared to molecular‐ion products is small in this energy range. This is in no way inconsistent with trajectory studies and may indeed be present at lower energies where it would be effectively concealed. The absolute characterization of mechanism as “direct” or “complex formation” does...