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

Authors: B. Mehnen, H. Hendaui, P. S. Zuchowski

DOI: https://doi.org/10.1093/mnras/stae1824

Date: 09/08/2024

Working Group: WG1, WG5

Grant Period: 2

Grant Period Goal (number): GAPG-7

Covered deliverables from the MoU (number): 3.3.2, 3.6.4, 5.3.1

Countries involved: Poland, Tunisia

Number of female/young/ITC coauthors: 2

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.