State-resolved electron capture in slow Ar²⁺-Ar/N₂ collisions

As a fundamental process in atomic physics, charge exchange relies on quantum state-resolved data that is crucial for fields such as astrophysics and plasma physics. However, there remains a gap in research on multi-electron target systems. This study aims to investigate the dynamic mechanisms of single/double electron capture in collisions between Ar²⁺ ions and Ar atoms or N₂ molecules at an energy of 40 keV, thereby supplementing high-precision experimental data in this field. The experiment was conducted on the Electron Beam Ion Source (EBIS) platform at the Institute of Modern Physics, Chinese Academy of Sciences, using the Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS)/reaction microscopes technique. An ion beam containing ground-state Ar²⁺(3s²3p⁴: ³P) and metastable Ar²⁺ (3s²3p⁴: ¹D, ¹S) was used as the projectile, colliding with a supersonic Ar/N₂ mixed gas target. Three-dimensional momentum of recoil ions was reconstructed through coincidence measurements of recoil ions and scattered ions, and the Q-value and scattering angle distribution were calculated. Theoretical comparisons were performed using the Molecular Coulombic Barrier Model (MCBM). Current experimental results indicate that the single-electron capture state populations of the two systems exhibit similarities but differ in contribution ratios: the Q-value spectrum of the Ar²⁺-Ar system contains an additional characteristic peak (corresponding to the process where the projectile ion captures an electron from the $3s$ orbital of the target while its own $3s$ electron is excited to the $3p$ orbital). In contrast, this characteristic peak is absent in the Ar²⁺-N₂ system due to the easy dissociation of excited N₂⁺ ions. For double-electron capture, both systems are dominated by capture to the ground state, but only the Ar²⁺-N₂ system shows a significant contribution from excited state populations. Comparison of scattering angles reveals that the higher the capture state of the product ion, the larger the corresponding scattering angle and the smaller the impact parameter. This is presumably because electron interactions become more complex at smaller impact parameters, leading to a higher probability of capture to high-energy levels. In the double-electron capture of the Ar²⁺-N₂ system, only the ground-state channel is populated at small angles (0–1.2 mrad). Additionally, electron capture exhibits a dependence on the impact parameter: as the angle increases (i.e., the impact parameter decreases), the Q-value of the capture reaction becomes smaller, and the reaction tends to be more endothermic.