Rotationally inelastic rate coefficients from collisions with H2 for the recently sighted OCN- (NCO-) anion

Abstract

The cyanate anion (equivalently noted as NCO$^-$ or OCN$^-$) is a species of considerable astrophysical relevance and, after years of searching, has been recently detected as embedded in interstellar ices present in young stellar objects. Although it has not yet been detected elsewhere, i.e. in the denser gas of the interstellar medium (ISM), we already know that sputtering and desorption of adsorbed molecular species from ices are important processes extensively discussed in the literature. Hence, to glean specific information on its collision-induced rotational state-changing processes, and therefore provide indications on its likely distribution between rotational states in the gas phase, we generate a new ab initio potential describing the interaction of this simple anion with H$_2$, one of the most abundant partners in that ISM environment. We then employ a reduced-dimensionality potential energy surface [a two-demensional (2D) interaction] to generate fully CC (coupled channel) 2D quantum scattering calculations to obtain state-to-state rotationally inelastic cross-sections, for collision energies in the range of 10$^{-4}$ to 1000 cm$^{-1}$. We further obtain the corresponding inelastic rate coefficients as a function of temperature from 5 to 150 K. The results are also compared with the results obtained earlier for He. The consequences of our findings on their non-equilibrium (non-local thermodynamic equilibrium) rotational populations in gas-phase interstellar environments are illustrated via analysis of their gas-phase critical densities.