State-resolved electron capture in low-energy Ar<sup>2+</sup>-Ar/N<sub>2</sub> collisions

<sec>As a fundamental process in atomic physics, charge exchange relies on quantum state-resolved data that is crucial for various fields such as astrophysics and plasma physics. However, there remains a gap in the research on multi-electron target systems. This study aims to investigate the dynamic mechanisms of single/double electron capture in collisions between Ar<sup>2+</sup> ions and Ar atoms or N<sub>2</sub> molecules at an energy of 40 keV, thereby supplementing high-precision experimental data in this field. The experiment is 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) technique. An ion beam containing ground-state Ar<sup>2+</sup> (3s<sup>2</sup>3p<sup>4</sup> <sup>3</sup>P) and metastable Ar<sup>2+</sup> (3s<sup>2</sup>3p<sup>4</sup> <sup>1</sup>D, <sup>1</sup>S) is used as the projectile, colliding with a supersonic Ar/N<sub>2</sub> mixed gas target. Three-dimensional momentum of recoil ions is reconstructed through coincidence measurements of recoil ions and scattered ions, and the <i>Q</i>-value and scattering angle distribution are calculated. Theoretical comparisons are performed using the molecular Coulombic over barrier model (MCBM).</sec><sec>The results show that there are similarities in the populations of single-electron captured states between the two systems, but the contribution ratios are different: the <i>Q</i>-value spectrum of the Ar<sup>2+</sup>-Ar system contains an additional characteristic peak, which corresponds 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 Ar<sup>2+</sup>-N<sub>2</sub> system due to the easy dissociation of excited <inline-formula><tex-math id="M12">\begin{document}$ \text{N}_{2}^{+} $\end{document}</tex-math></inline-formula> ions. For double-electron capture, both systems are dominated by capturing electrons to the ground state, but only the Ar<sup>2+</sup>-N<sub>2</sub> system shows a significant contribution from excited state populations. The comparison of scattering angles reveals that the higher the capture state of the product ion, the larger the corresponding scattering angle is and the smaller the impact parameter is. This is presumably because electron interactions become more complex at smaller impact parameters, leading to a higher probability of capturing electrons to high-energy levels. In the double-electron capture of the Ar<sup>2+</sup>-N<sub>2</sub> system, only the ground-state channel is populated at small angles (0–1.2 mrad). Additionally, electron capture exhibits dependence on impact parameter: as the angle increases (i.e. the impact parameter decreases), the <i>Q</i>-value of the capture reaction decreases, indicating that the reaction tends to be more endothermic.</sec>