Highly excited quantum state-selective capture dynamics in slow Ar8+−H2 collisions

We report on the combined experimental and theoretical studies of the single-electron capture collisions of Ar^8+ projectiles with the H₂ molecules at 1, 2, and 4 keV/u collision energies. The nondissociative recoil H₂^+ molecular ions are measured in coincidence with the charge-changing Ar^7+ projectiles. The relative cross sections of the different state-selective capture channels are obtained from the experimentally measured Q-value spectra. The 1s electron transfers to the highly excited 4d+4f, 5s, 5p, 5d+5f+5g, 6s+6p, and 6d+6f+6g+6h states are resolved experimentally. The differential scattering angle distributions for the dominant 1s to 5s, 5p, and 5d+5f+5g transitions are compared with the two-center atomic orbital close-coupling methods. A collision energy-dependent dynamical coupling effect is also observed for the 1s5p₀ and 1s5p₁ transitions. The dominant oscillatory structures in the scattering-angle distributions are attributed to Stueckelberg-type oscillations. In contrast, the less visible undulations in the smaller scattering angles imprint the signature of the quantum matter-wave scattering of the projectiles. The quantum nature of the oscillations in the angular distributions is further validated by classical calculations. Our study thus illustrates the highly excited quantum state-selective electron capture process and sheds light on the scattering-angle-range-dependent collision dynamics for highly charged ion-molecule collisions in the highly perturbative regime.