Quantum rotational dynamics of -C(Σ-g) by H at low temperatures employing a machine learning augmented potential energy surface.

A new four dimensional (4D) potential energy surface (PES) is generated for the collision of C(Σ) with H(Σ), considering both molecules as rigid rotors. A supervised neural network model is created to augment the PES and to get the missing data points. Furthermore, space fixed expansion of the augmented PES is carried out using a least squares fit over two spherical harmonics terms, resulting in radial coefficients (, , and ). The centre of symmetry in both C and H forces and to have even values, respectively. Moreover, the rotational states of C are only populated by odd levels due to its ground state triplet symmetry and the nuclear spin ( = 0) of C. The cross-sections and rate coefficients with and H partners are studied for various odd state transitions, where the rate coefficients of the are 10-20% higher than those of the latter. The de-excitation rates obtained by the H collisions are also compared to those of He and are found to be ∼1.7-2.8 times the He rates, across various order transitions. The simple scaling of He rates using a factor of 1.38 proves insufficient to describe H rates. Therefore, these results show the importance of explicitly studying H as an important colliding partner, governing the kinetics of various rotational processes in the interstellar space.