Computed Rotational Collision Rate Coefficients for Recently Detected Anionic Cyanopolyynes

We report new results from quantum calculations of energy-transfer processes taking place in interstellar environments and involving two newly observed molecular species: C$_5$N$^-$ and C$_7$N$^-$ in collision with He atoms and the p-H$_2$ molecules. These species are part of the anionic molecular chains labeled as cyanopolyynes which have been observed over the years in molecule-rich Circumstellar Envelopes and in molecular clouds. In the present work, we first carry out new $ab$ $initio$ calculations for the C$_7$N$^-$ interaction potential with He atom and then obtain state-to-state rotationally inelastic cross sections and rate coefficients involving the same transitions which have been observed experimentally by emission in the interstellar medium (ISM) from both of these linear species. For the C$_5$N$^-$/He system we extend the calculations already published in our earlier work (see reference below) to compare more directly the two molecular anions. We extend further the quantum calculations by also computing in this work collision rate coefficients for the hydrogen molecule interacting with C5N$^-$, using our previously computed interaction potential. Additionally, we obtain the same rate coefficients for the C$_7$N$^-$/H$_2$ system by using a scaling procedure that makes use of the new C$_7$N$^-$/He rate coefficients, as discussed in detail in the present paper. Their significance in affecting internal state populations in ISM environments where the title anions have been found is analyzed by using the concept of critical density indicators. Finally, similarities and differences between such species and the comparative efficiency of their collision rate coefficients are discussed. These new calculations suggest that, at least for the case of these longer chains, the rotational populations could reach local thermal equilibrium conditions within their observational environments

Computed Rotational Collision Rate Coefficients for Recently Detected Anionic Cyanopolyynes

We report new results from quantum calculations of energy-transfer processes taking place in interstellar environments and involving two newly observed molecular species: C _5 N ^− and C _7 N ^− in collision with He atoms and p–H _2 molecules. These species are part of the anionic molecular chains labeled as cyanopolyynes, which have been observed over the years in molecule-rich circumstellar envelopes and in molecular clouds. In the present work, we first carry out new ab initio calculations for the C _7 N ^− interaction potential with He atoms and then obtain state-to-state rotationally inelastic cross sections and rate coefficients involving the same transitions, which have been observed experimentally by emission in the interstellar medium (ISM) from both of these linear species. For the C _5 N ^− /He system, we extend the calculations already published in Biwas et al. to compare more directly the two molecular anions. We extend further the quantum calculations by also computing in this work collision rate coefficients for the hydrogen molecule interacting with C _5 N ^− , using our previously computed interaction potential. Additionally, we obtain the same rate coefficients for the C _7 N ^− /H _2 system by using a scaling procedure that makes use of the new C _7 N ^− /He rate coefficients, as discussed in detail in the present paper. Their significance in affecting internal state populations in ISM environments where the anionic cyanopolyynes have been found is analyzed by using the concept of critical density indicators. Finally, similarities and differences between such species and the comparative efficiency of their collision rate coefficients are discussed. These new calculations suggest that, at least for the case of these longer chains, the rotational populations could reach local thermal equilibrium conditions within their observational environments

Rotational State-Changing Rate Coefficients Computed for Recently Detected Anionic Cyanopolyines C5N− and C7N− in Collision with He [Dataset]

[EN]We report quantum calculations involving the dynamics of energy-transfer processes taking place in Interstellar environments and involving the title molecular species which are part of the anionic molecular chains labeled as cyanopoyines which have very recently been observed in molecule-rich Circumstellar Envelopes. We first carried out new ab initio calculations for the C 7 N − involving its interaction potential with He atoms and therefore obtained state-to-state rotationally inelastic cross sections and rate coefficients over the relevant range of temperatures for both long chains and regarding the state-changing processes which have been very recently observed as radiative transitions for either molecule. The similarities and differences between such species and the comparative efficiencies of collisional rate coefficients with radiative coefficients are analysed and discussed.f Their significance on affecting internal state populations in ISM environments where the title anions are present is discussed.The original raw points from the ab initio calculations. The multipolar coefficients for the Legendre expansion of the rigid rotor potential for the C7N− /He system. The computed inelastic cross sections. The rate coefficients obtained from the 2D-RR-PES for the C7N− anion. The critical density values obtained for both anions and linked to their collision data for either He or H2