Electron-impact rotational and hyperfine excitation of HCN, HNC, DCN and DNC

Abstract

Rotational excitation of isotopologues of HCN and HNC by thermal electron-impact is studied using the molecular {\bf R}-matrix method combined with the adiabatic-nuclei-rotation (ANR) approximation. Rate coefficients are obtained for electron temperatures in the range 5$-$6000 K and for transitions among all levels up to J=8. Hyperfine rates are also derived using the infinite-order-sudden (IOS) scaling method. It is shown that the dominant rotational transitions are dipole allowed, that is those for which $\Delta J=1$. The hyperfine propensity rule $\Delta J=\Delta F$ is found to be stronger than in the case of He$-$HCN collisions. For dipole allowed transitions, electron-impact rates are shown to exceed those for excitation of HCN by He atoms by 6 orders of magnitude. As a result, the present rates should be included in any detailed population model of isotopologues of HCN and HNC in sources where the electron fraction is larger than 10$^{-6}$, for example in interstellar shocks and comets.