Abstract

The HCN molecule is among the main tracers of the interstellar medium (ISM) and the observation of its isotopologue DCN should help to understand the physical properties and chemical history of the molecular clouds. We analyse the effects of the substitution of H by D in the collision of HCN with He. The bound levels of the DCN-He complex are reported and compared with the HCN-He levels. The close-coupling equations were solved in the space-fixed frame using the potential energy surface recently published for the HCN-He system. These calculations were performed with two different approaches in which the triatomic molecule was modelized as a rigid rotor or as a rigid bender, the latter approach allowing us to investigate rovibrational transitions. Rovibrational rate coefficients of DCN in collision with He were computed and compared with those for HCN-He. The ratio of the rate coefficients between both isotopologues varies from 0.4 to 3.9 for rotational transitions in the ground vibrational level. In the case of the first l-type doubling transition, we found that the rate coefficients have the same order of magnitude than those for pure rotational transitions. Therefore, in regions of ISM where the first vibrationally excited level of DCN can be populated, l-type doubling transitions are expected to be as important as pure rotational transitions.