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本文采用反应显微成像谱仪研究了炮弹能量40 keV条件下Ar$^{2+}$离子与Ar与N$_{2}$气体碰撞体系的单电子俘获和双电子俘获动力学过程。通过高精度动量成像技术，实验成功测定了Ar$^{2+}$-Ar和Ar$^{2+}$-N$_{2}$碰撞体系中态分辨的单电子和双电子俘获截面，并获取了炮弹离子的散射角分布。实验数据分析表明：在单电子俘获过程中，Ar$^{2+}$与Ar原子作用时，电子主要被俘获至炮弹$3s3p^{6}$激发态；而与N$_{2}$分子相互作用时，电子优先占据$3s^{2}3p^{5}$态。值得注意的是，Ar原子体系的单电子俘获截面分布与分子库仑过垒模型预测的反应窗口呈现良好的一致性。在双电子俘获过程中，无论靶介质为Ar原子还是N$_{2}$分子，双电子俘获均以基态（$3s^{2}3p^{6}$）占据主导地位。此外，还对Ar$^{2+}$-Ar/N$_{2}$的单、双俘获过程的散射角分布进行了分析和定性解释。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 Ar$^{2+}$ ions and Ar atoms or N$_{2}$ 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 Ar$^{2+}$($3s^{2}3p^{4}$: $^{3}$P) and metastable Ar$^{2+}$ ($3s^{2}3p^{4}$: $^{1}$D, $^{1}$S) was used as the projectile, colliding with a supersonic Ar/N$_{2}$ 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 Ar$^{2+}$-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 Ar$^{2+}$-N$_{2}$ system due to the easy dissociation of excited N$_{2}^{+}$ ions. For double-electron capture, both systems are dominated by capture to the ground state, but only the Ar$^{2+}$-N$_{2}$ 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 Ar$^{2+}$-N$_{2}$ 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.-
Keywords:
- low-energy heavy ions /
- charge exchange /
- state-selective cross-sections /
- angular differential cross-sections /
- reaction microscopes
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