A quantum chemical study of the N 2H + + e − → N 2 + H reaction I: The linear dissociation path

Abstract

A theoretical investigation of the dissociative recombination (DR) of linear N 2 H + ( X 1 Σ g + ) to give N 2 + H has been undertaken because it is of interest for astrochemistry and also because it has been recently studied experimentally. Using state of the art quantum chemical methods, it is shown that the lowest 2Σ repulsive state of N 2H leading to the N 2 and H fragments in their ground electronic states does not cross the curve of the ion nor the one of the lowest N 2H Rydberg state. This lowest 2Σ repulsive state is very low in energy. Its curve passes below the 1Σ N 2H + state and below the lowest bound 2Σ N 2H states. However, it is also shown that there exist higher repulsive 2Σ and 2Δ states of N 2H (the second and third repulsive states) crossing the ion curve. These states will lead to the formation of N 2 in its 3 Σ u + and 3Δ u states. This study, the first of its type, shows that the DR of linear N 2H + should involve the direct mechanism and that it should lead to N 2 in its first excited states. However this process may not be efficient for N 2H + in its ground vibrational state ( v = 0), a state in which it exists in the cold environment of the interstellar medium. For the DR to be efficient for N 2H + in its ground v = 0 vibrational state, bent geometries of the ion might have to be considered.