Quantum rotational dynamics of linear C₅ at low interstellar temperatures for H₂ collision.

This article focuses on the quantum collisional dynamics of the C₅ carbon chain molecule with para- and ortho-hydrogen (p-H₂ and o-H₂) in the interstellar medium (ISM), aiming to improve modeling of its abundance under non-local thermodynamic equilibrium conditions. A four-dimensional potential energy surface (PES) for the C₅–H₂ interaction was computed using high-level ab initio methods and augmented via a neural network model to achieve spectroscopic accuracy. The PES was expanded into bispherical harmonics to perform close-coupling calculations, yielding rotational excitation cross sections and rate coefficients for C₅ collisions with both p-H₂ and o-H₂ up to 100 K. Results show that o-H₂ collisional rates are generally 10%–20% higher than p-H₂ rates for Δj = 2 transitions, reflecting the influence of quadrupole–quadrupole interactions, and that these rates differ notably from those involving helium collisions. The study provides essential collisional data to support future astronomical detections and abundance estimations of C₅ in the ISM.