Production of H 3 + ions in low-energy collisions between hydrogen molecular ions

The effective cross section for the H ₂ ⁺ + H ₂ ⁺ → H ₃ ⁺ + p reaction in the energy range 5.7–11.5 eV is measured by the split beam method. The cross-section maximum at an energy of ～8 eV is related to the production of the H ₄ ⁺⁺ compound system. The reaction threshold W _thr ≈5 eV provides evidence in favor of the classical model with the H ₂ ⁺ ion charge fixed on one of the two nuclei during the entire collision event.