Subshell-resolved electron capture in O4+ -He collisions near Bohr velocity

Subshell-resolved single-electron capture has been investigated in collisions between O⁴⁺ ions and He atoms at energies from 7 to 70 keV/nucleon, using high-resolution cold-target recoil-ion momentum spectroscopy. Analysis of the Q-value spectra, n-shell dependence, relative branching ratios, and angular distributions provides insight into the dominant reaction channels. Capture into the $n=3$ state remains dominant in the entire projectile energy, agreeing with predictions from the molecular Coulombic over barrier model. At low energies, capture preferentially populates the $3s$ subshell, while higher l states like $3d$ become dominant at higher projectile energies. The oscillatory transverse momentum distribution of $3s$ states is attributed to de Broglie wave Fraunhofer diffraction, indicative of the quantum nature of the collision dynamics.