Electron-impact rotational and hyperfine excitation of HCN, HNC, DCN and DNC
Résumé
Rotational excitation of isotopologues of HCN and HNC by thermal electron-impact is studied using the molecular R-matrix method combined with the adiabatic-nuclei-rotation 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 scaling method. It is shown that the dominant rotational transitions are dipole-allowed, that is, those for which ΔJ = 1. The hyperfine propensity rule ΔJ = ΔF is found to be stronger than that 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 six 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.
Domaines
Planète et Univers [physics]
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