title: Fine and hyperfine excitation of NH and ND by He: on the importance of calculating rate coefficients of isotopologues.
abstract: The NH and ND molecules play an important role in interstellar nitrogen chemistry. Accurate modeling of their abundance in space requires the calculation of rates for collisional excitation by the most abundant interstellar species. We calculate rate coefficients for the fine and hyperfine excitation of NH and ND by He. State-to-state rate coefficients between the first levels of NH and ND were obtained for temperatures ranging from 5 to 150 K. Fine structure resolved rate coefficients present a strong propensity rule in favor of Δj = ΔN transitions, as expected from theoretical considerations. The Δj = ΔF(1) = ΔF propensity rule is observed for the hyperfine transitions of both isotopologues. The two sets of fine structure resolved rate coefficients are compared in detail and we find significant differences between the two isotopologues. This comparison shows that specific calculations are necessary for the deuterated isotopologues of any hydride. The new rate coefficients will help significantly in the interpretation of NH and ND terahertz spectra observed with current and future telescopes, and enable these molecules to become a powerful astrophysical tool for studying the nitrogen chemistry.
authors.affiliation: LOMC-UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP 540, 76058, Le Havre, France.
authors.forename: Fabien
authors.initials: F
authors.lastname: Dumouchel
authors.forename: Jacek
authors.initials: J
authors.lastname: Kłos
authors.forename: Robert
authors.initials: R
authors.lastname: Toboła
authors.forename: Aurore
authors.initials: A
authors.lastname: Bacmann
authors.forename: Sébastien
authors.initials: S
authors.lastname: Maret
authors.forename: Pierre
authors.initials: P
authors.lastname: Hily-Blant
authors.forename: Alexandre
authors.initials: A
authors.lastname: Faure
authors.forename: François
authors.initials: F
authors.lastname: Lique