Quantum dynamics study of rotational transitions of NCCN induced by He collision

Abstract

Quantum dynamics of the molecule cyanogen (NCCN) and its collision with helium taking place in the interstellar medium has been studied. An ab initio potential energy surface of NCCN-He, a van der Waals complex, is generated using the high-level single reference coupled-cluster with single and double and perturbative triple excitation method and aug-cc-pVQZ basis sets. Using the multipole expansion, Legendre coefficients have been calculated and utilized in determining collisional cross sections. Close-coupling calculations have been performed to study rotational excitations for He collision with NCCN. Due to nuclear spin statistics, collision induced transitions have even Δj, while odd Δj transitions are forbidden. The presence of resonances arising from rapid oscillation of cross sections in the low energy region is the result of quasi-bound states in the NCCN-He van der Waals complex. Among all the transitions, Δj = 2 are found to be predominant for excitation. Thereafter, for each transition, the rate coefficients have been calculated which decrease with increasing values of j and Δj. The result of this work will be helpful to accurately model the abundance of cyanogen in stellar atmospheres and interstellar gas.