A quantum chemical study of the N2H+ + e− → N2 + H reaction I: The linear dissociation path

A theoretical investigation of the dissociative recombination (DR) of linear N2H+(X1Σg+) to give N2 + 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 N2H leading to the N2 and H fragments in their ground electronic states does not cross the curve of the ion nor the one of the lowest N2H Rydberg state. This lowest 2Σ repulsive state is very low in energy. Its curve passes below the 1Σ N2H+ state and below the lowest bound 2Σ N2H states. However, it is also shown that there exist higher repulsive 2Σ and 2Δ states of N2H (the second and third repulsive states) crossing the ion curve. These states will lead to the formation of N2 in its 3Σu+ and 3Δu states. This study, the first of its type, shows that the DR of linear N2H+ should involve the direct mechanism and that it should lead to N2 in its first excited states. However this process may not be efficient for N2H+ 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 N2H+ in its ground v = 0 vibrational state, bent geometries of the ion might have to be considered.