Hyperfine excitation of NH and ND by molecular hydrogen

Rate coefficients and astrophysical modeling

¹ Univ. Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France
e-mail: paul.pirlot@univ-rennes.fr; francois.lique@univ-rennes.fr
² Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain

Received: 6 December 2023
Accepted: 8 January 2024

Abstract

The NH and ND radicals are of key importance in the comprehension of nitrogen chemistry and the enhancement of deuterated molecules in the interstellar medium. Observations by space telescopes yield spectra that can resolve the fine and hyperfine structure of these radicals, a consequence of the electronic and magnetic interactions of nitrogen, hydrogen, and deuterium nuclei. Accurate rate coefficients, induced by collisions with H₂, are required to interpret spectra of these radicals. We report the first rate coefficients for fine and hyperfine transitions of NH and ND in collision with both ortho- and para-H₂. Based on a recent four-dimensional potential energy surface, fine-structure resolved cross sections and rate coefficients are computed with the time-independent close-coupling method over a temperature range of 5–300 K. Our calculations include the first 25 energy levels of NH and ND. Hyperfine resolved cross sections and rate coefficients are determined using the infinite-order sudden (IOS) approximation between 5 and 200 K for NH and 100 K for ND. We consider the first 71 and 105 energy levels of NH and ND, respectively. General propensity rules are discussed. We found a significant isotopic substitution effect in the rate coefficients. In addition, the rate coefficients for collisions with H₂ are larger than those with He by a factor of up to 5, leading to lower critical densities for collisional excitation with H₂ than He. The impact of the new set of collisional data has been investigated in simple radiative transfer models of the NH emission seen toward the Orion Bar and the ejecta of the η Carinae binary star. We observed significant differences by a factor of 5 between the presently determined column densities for NH compared to those from the literature using He as a collider.