Molecular beam and trajectory studies of reactions of H+ with H2

We have employed two complementary techniques, molecular beam experiments and ``trajectory surface hopping'' theory, to investigate proton‐hydrogen molecule scattering at relative energies between 1 and 7 eV. Absolute cross sections, product translational energy distributions, and product velocity contour diagrams have been obtained from both theory and experiment for the H⁺ + D₂ and D⁺ + HD isotope arrangements. Agreement is excellent, particularly for a theory which involves no empirical information and no adjustable parameters. By combining results from experiment and theory we have been able to develop a fairly complete, reliable, and simple picture of the dynamics of this elementary collision process. For relative energies below about 3 eV the reaction involves a short‐lived collision intermediate, whereas above 4 eV it proceeds by a predominantly direct, impulsive mechanism. Above 5.5 eV momentum transfer is well represented by a hard‐sphere collision model involving only short‐range repulsive forces. The avoided intersection of the two lowest singlet potential energy surfaces of ${\mathrm{H}}_3^{+}$ is of primary importance in determining the partitioning of energy among products, the likelihood of electronic transitions, the angular and velocity distributions of products, and the competition among various rearrangement, charge transfer, and fragmentation channels.