Rotational excitation and de-excitation of the interstellar propargyl (H2CCCH+) cation by collisions with helium atoms

ABSTRACT The propargyl cation, H2CCCH+, is a crucial hydrocarbon precursor in the chemical evolution of the interstellar medium (ISM). It serves as a key intermediate for understanding ISM hydrocarbon chemistry. This work presents state-to-state collisional rate coefficients for rotational excitation and de-excitation of ortho- and para-H2CCCH+ colliding with He at ISM-relevant temperatures (T ≤ 60 K). We employed a high-level post-Hartree-Fock CCSD(T)-F12/aug-cc-pVTZ method to generate a 3D interaction potential energy surface (3D-PES) for the weakly bound H2CCCH+-He complex. The analytical 3D-PES was then used in close-coupling calculations to determine cross-sections for rotational excitation and de-excitation of ortho- and para-H2CCCH+ by collisions with He for kinetic energies (Ek) up to 300 cm−1. Inelastic collision rate coefficients were subsequently obtained by thermally averaging the cross-sections over a Maxwell–Boltzmann distribution of kinetic energies. Our results indicate that at very low temperatures (T < 15 K), rotational transitions associated with Δj = Δkc = ±1, which are crucial for the identification of H2CCCH+ in astronomical surveys, exhibit the largest cross-sections. In contrast, at higher temperatures, transitions with Δj = Δkc = ±2 become more dominant. These data hold significant implications for the determination of the H2CCCH+ abundance in the ISM and improve the modelling of interstellar hydrocarbon-chain reaction pathways.