Collisional excitation of N+ (3 P) in interstellar clouds

The detection of the N+ ion in ionized interstellar regions, as well as its use as a tracer for atomic nitrogen (N) in interstellar media, makes it an interesting ion from the astrophysical point of view. Accurate determination of the N+ abundance in the interstellar clouds requires detailed modelling of its excitation conditions. Hence, collisional excitation induced by the dominant interstellar species has to be studied. In this paper, we study the collisional excitation of N+ by He and H-2. We provide new potential energy surfaces describing the interactions between N+ and He and between N+ and H-2 considered as a structureless collider. Quantum calculations of the excitation cross-sections for collisions of N+ with He and H-2 are provided, as well as the first excitation rate coefficients for N+ induced by these two species. The results are compared to collisional data for the excitation induced by electrons. It is shown that the excitation of N+ by a neutral collider can play a significant role in weakly ionized regions as well as in warm molecular clouds.