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Context. This work is motivated by the necessity to take account of both the nuclear spin symmetries of H₂ and the spin-orbit interaction of N⁺ ions in order to investigate gas phase reactions in interstellar chemistry, leading to the formation of nitrogenous and deuterated compounds.
Aims. The main objective in this work is to determine the rate coefficients for each possible initial quantum state of the reactants N⁺ (³P_j) + H₂ (J) (and their isotopic variants). Only in this way does it become possible both to analyse experimental data and to develop realistic applications to interstellar chemical models to constrain the gas phase chemistry of ammonia and its isotopologues.
Methods. A statistical treatment is presented of state selective reactive collisions involving N⁺ ions in fine structure state j with H₂ or HD molecules in a rotation level J of the ground vibration state, leading either to the production of NH⁺ ions and H in the case of the H₂ reactant, and to the production of either NH⁺ ions or ND⁺ in the case of the HD reactant. The energies of fine structure states (j=0,1,2) of the N⁺ ions are treated on an equal footing with the other energies of internal motions. All fine structure states are considered to be reactive.
Results. Cross sections for state-to-state collisions are calculated for collision energies ranging from 0.1-30meV. These cross sections are then averaged over the kinetic energies of the reactants for each (J,j) to obtain the rate coefficients for a range of kinetic temperatures 10-200K. The exo/endothermicity of the reactions involving N⁺ (³P_j) + H₂ (J) (and isotopic variants) is derived from the difference ΔE_e between the dissociation energies of the electronic molecular potentials of NH⁺ and H₂. The value ΔE_e=101meV is found to satisfactorily reproduce the experiments performed with ortho-H₂ and to a lesser extent with para-H₂. This value is used to determine the rate coefficient of the N⁺ + HD reaction leading to the formation of ND⁺. The calculated value is consistent with the available experimental data.
Conclusions. The present results allow for the determination of reaction rate coefficients for any given distribution of specific fine structure and rotational state populations of the reactants. In interstellar conditions, where N⁺ is in its ³P₀ state and para- and ortho-H₂ respectively in J=0 and J=1. Our results enable a study of the influence of the ortho/para evolution of molecular hydrogen on the formation of nitrogen compounds.