Interatomic Coulombic electron capture in H⁺-Li and H⁺-He-Li systems: The role of He as an intermediate species

Interatomic Coulombic electron capture (ICEC) is an environment-assisted electron capture mechanism where a free electron efficiently attaches to a quantum system by transferring its excess energy to a neighboring atom or molecule, thereby ionizing it. In this study, we explore electron attachment in the H^+-Li and H^+-He-Li systems, emphasizing the role of the intermediate He species in the ICEC process. The calculations are performed using the ab initio R-matrix method, combined with the configuration interaction method to account for electronic correlation. Convergence is tested by varying active spaces and the number of partial waves in the Legendre expansion for the one-electron and two-electron integrals. When the ICEC process involves He^- as a virtual intermediate state, our results show that He can significantly enhance ICEC efficiency. In contrast, when He acts merely as a geometric intermediate without participating as a virtual state, its presence has little effect on the direct ICEC process between H^+ and Li. Pronounced resonance features are observed in the ICEC cross sections, and the partial contributions are analyzed in detail.