Rotational state-changes in C5N− by collisions with He and H2.

The anion C₅N⁻ is one of the largest linear (C,N)-bearing chains detected in the interstellar medium. Here we present and discuss the general features of new ab initio potential energy surfaces describing the interaction of this linear anion with He and H₂. We employ a Legendre Polynomials expansion representation for the former and an artificial neural network fit for the latter. We then carry out quantum scattering calculations to yield rotationally inelastic cross-sections for collisions with He and H₂, using relative translational energy values in the range of 0.1–300 cm⁻¹. We then obtained the corresponding inelastic rate coefficients as a function of temperature covering the range from 1 to 100 K. The results for these two systems are compared with each other, as well as with the earlier results on the C₃N⁻ colliding with the same partners. We found that the final inelastic rate coefficients for this anion are all fairly large, those from collisions with H₂ being the largest. The consequences of such findings on their non-equilibrium rotational populations in interstellar environments are discussed in our conclusions. [ABSTRACT FROM AUTHOR]