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

TL;DR

This study calculates electron-impact excitation rates for isotopologues of HCN and HNC using advanced quantum methods, revealing their significance in astrophysical environments with high electron fractions.

Contribution

It provides the first comprehensive set of electron-impact excitation rates for HCN and HNC isotopologues, including hyperfine transitions, across a wide temperature range.

Findings

Electron-impact rates exceed He-induced excitation rates by 6 orders of magnitude.

Dipole allowed transitions dominate rotational excitation.

Hyperfine propensity rule ΔJ=ΔF is stronger than in He collisions.

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.