Chemical pathways and kinetic rates of the N(4S) + N2 → N3 solid phase reaction: could the N3 radical be a temperature sensor of nitrogen ices in dense molecular clouds?

International audienceEven though the N 3 radical has not yet been detected in the interstellar medium, its formation still remains a challenge. For a long time, bombardments of N 2 ices by energetic particles were the only way to from the azide radical as it was thought that ultraviolet (UV) photons were not strong enough to fragment the molecular nitrogen into N atoms. Consequently, it had been suggested that N 3 could be used as discriminator between ice radiolysis and ice photolysis until a very recent study that has shown that photodecomposition of molecular nitrogen by UV photons might also be a source of the azide radical. In contrast to all these nitrogen ice bombarding experiments, only two laboratory studies have investigated the N 3 formation where the reactants N and N 2 mixed in the gas phase were co-condensed at 12 K, and this raised a new question concerning whether N + N 2 → N 3 took place in the solid phase or in the gas phase. The experimental results of these two studies are contradictory and the problem of the characterization of N 3 formation by co-condensing atomic N and molecular N 2 has persisted to the present day. In this paper, we give a clear answer to this question, by investigating the kinetic rates of the N(4 S) + N 2 → N 3 reaction in the solid phase in the temperature range of 3–35 K. We find a rate constant of 7.7 × 10 −23 s −1 molecule −1 cm 3 for the azide radical formation in the solid phase and we provide new information on the N 3 infrared signature, which could be used to characterize the temperature and the structure of nitrogen ices