State-selected reactive scattering. I. H + 2+H2 --> H + 3+H

Cross sections for the reaction H⁺₂+H₂→H⁺₃+H, differential in scattering angle and recoil energy, are measured in a molecular-beam experiment at c.m. collision energies of 1.5, 2.3, 3.5, and 5.3 eV. Resonantly enhanced four-photon ionization prepares H⁺₂ in selected vibrational-state distributions, allowing a systematic exploration of the effects of reactant energy on the product angular and energy distributions. Angular data are interpreted on the basis of competition between H⁺₃ formation and collision-induced dissociation. The nominal atom-transfer (AT) and proton-transfer (PT) processes are identified respectively with forward and backward scattered H⁺₃. Effects of reactant energy on AT and PT cross sections in H⁺₂+H₂ are compared with previous observations on D⁺₂+H₂ and H⁺₂+D₂. The fraction of the available energy appearing as H⁺₃+H recoil ranges from 26% to 39% depending on reactant conditions. Previous surface-hopping trajectory calculations successfully predict most of the observed trends. Evidence is given for the existence of long-lived H⁺₃ with internal energy in excess of the dissociation limit.