Dissociative recombination and ion-pair formation in HeH^+ isotopologues: A time-dependent wave-packet study including rotational coupling

We present a comprehensive theoretical investigation of dissociative recombination (DR) and resonant ion-pair (RIP) formation in HeH^+ isotopologues using time-dependent wave-packet propagation methods. Nuclear dynamics are treated on a set of 23 coupled electronic states, including ²Σ, ²Π, and ²Δ symmetries, in both adiabatic and strictly diabatic representations, with rotational couplings explicitly included. Reaction cross sections are computed over collision energies ranging from 0 to 50 eV. The results reveal that inclusion of a large manifold of resonant states and rotational couplings significantly enhances the DR cross section relative to earlier theoretical studies. In the diabatic representation, ²Σ states dominate the recombination dynamics, while in the adiabatic representation, ²Π and ²Δ states contribute significantly at low collision energies. For RIP formation, two different diabatization schemes yield systematically larger cross sections than previous models, highlighting the sensitivity of ion-pair production to electronic coupling structure. Isotopic effects are examined, showing a clear inverse dependence of cross section magnitude on reduced mass. The present results underscore the importance of multi-state coupling and nonadiabatic effects in accurately describing electron-molecule collision processes in primordial and astrophysical plasmas.