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

Area of Science:

• Astrochemistry
• Quantum Chemistry
• Interstellar Medium Physics

Background:

• Accurate modeling of carbon chain abundance in the interstellar medium (ISM) requires understanding their quantum dynamics under non-local thermodynamic equilibrium conditions.
• The C5 molecule is the longest pure carbon chain observed in the ISM, making its collisional properties critical for astrochemical models.
• Previous approximations for calculating collisional rates with para-hydrogen (p-H2) using helium (He) or reduced dimensionality are limited, and no approximations exist for ortho-hydrogen (o-H2).

Purpose of the Study:

• To perform a full quantum dynamical calculation of the rotational excitation of C5 by collisions with both para- (p-H2) and ortho- (o-H2) hydrogen.
• To develop and utilize an accurate potential energy surface (PES) for C5-H2 interactions suitable for rigorous quantum scattering calculations.
• To provide state-to-state cross sections and rate coefficients for rotational transitions of C5 relevant to ISM conditions.

Main Methods:

• Calculation of the C5-H2 potential energy surface (PES) using high-level ab initio theory (CCSD(T)-F12a/AVTZ).
• Development of a neural network model to accurately fit and augment the PES, ensuring spectroscopic accuracy.
• Expansion of the augmented PES into radial terms using bispherical harmonics and performing close-coupling quantum scattering calculations.

Main Results:

• Accurate state-to-state cross sections for rotational transitions in C5 upon collision with both p-H2 and o-H2 were obtained.
• Rate coefficients for various rotational transitions of C5 were computed, providing essential data for astrochemical models.
• The study establishes a robust computational framework for studying the collisional dynamics of larger carbon chains in the ISM.

Conclusions:

• The developed methodology allows for accurate quantum dynamical calculations of C5 rotational excitation by H2, overcoming limitations of previous approximations.
• The computed rate coefficients are vital for improving the accuracy of abundance calculations for C5 and other carbon chains in the ISM.
• This work paves the way for more comprehensive studies of interstellar carbon chemistry and the physical conditions in the ISM.