2021MNRAS.508..118D

Accurate estimates of the abundance of methylene (CH₂) in the interstellar medium require knowledge of both the radiative and collisional rate coefficients for the transfer of population between rotational levels. In this work, time-independent quantum close coupling calculations have been carried out to compute rate coefficients for the (de-)excitation of ortho- and para-CH₂ in collisions with ortho- and para-H₂. These scattering calculations have employed a recently computed, high-quality potential energy surface, based on the coupled cluster level of theory [RCCSD(T)-F12a], for the interaction of CH₂ in its ground \tildeX ³B₁ electronic state with H₂. The collisional rate coefficients were obtained for all fine-structure transitions among the first 22 and 24 energy levels of ortho- and para-CH₂, respectively, having energies less than 277 cm^–1. These rate coefficients are compared with previous calculated values, obtained by scaling data for CH₂-He. In the case of ortho-CH₂, whose levels display hyperfine structure, rate coefficients for transitions between hyperfine levels were also computed, by the M_J randomization approximation. Finally, some simple radiative transfer calculations are presented.