2018MNRAS.473.4824D

Accurate determination of the abundance of CH₂ in interstellar media relies on both radiative and collisional rate coefficients. We investigate here the rotational/fine-structure excitation of CH₂ induced by collisions with He. We employ a recoupling technique to generate fine-structure-resolved cross-sections and rate coefficients from close coupling spin-free scattering calculations. The calculations are based on a recent, high-accuracy CH₂-He potential energy surface computed at the coupled clusters level of theory. The collisional cross-section calculations are performed for all fine-structure transitions among the first 22 and 24 energy levels of ortho- and para-CH₂, respectively, and for temperatures up to 300 K. As a first application, we simulate the excitation of CH₂ in typical molecular clouds. The excitation temperatures of the CH₂ lines are found to be small at typical densities of molecular clouds, showing that the non-local thermodynamic equilibrium approach has to be used to analyse interstellar spectra. We also found that the fine-structure lines connected with the 4₀₄ - 3₁₃ and 5₀₅ - 4₁₄ rotational transitions show possible maser emissions so that they can be easily seen in emission. These calculations show that CH₂ may have to be detected mainly through absorption spectra.