Competition between dissociative and nondissociative single-electron capture in He²⁺-O₂ collisions

State-selective single-electron capture in slow collisions of He²⁺ ions with O₂ has been studied experimentally at laboratory impact energies between 0.1 and 1.0 keV by means of the translational energy-gain spectroscopy technique. The translational energy spectrum clearly shows that the dominant reaction channel is due to dissociative single-electron capture [single-electron capture into He⁺(n=1) with target ionization, i.e., transfer ionization] at low energies, whereas contributions from capture into the He⁺(n=2) state with the production of O₂⁺ in the ground state (X ²Π_g) (nondissociative single-electron capture) increase as the collision energy is increased and becomes the dominant process at collision energies E>~0.7 keV. The energy dependence of total cross sections for single-electron capture is also measured and found to slowly increase with increasing collision energies.