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The formation of hydrides by gas-phase reactions between H2 and a heavyelement atom is a very selective process. Reactions with ground-state neutralcarbon, oxygen, nitrogen, and sulfur atoms are very endoergic and have highenergy barriers because the H2 molecule has to be fragmented before a hydridebond is formed. In cold interstellar clouds, these barriers exclude theformation of CH, OH, NH, and SH radicals through hydrogen abstractionreactions. Here we study a very energetically unfavorable process, the reactionof N(4S) atoms with H2 molecules. We calculated the reaction rate coefficientfor H2 in different vibrational levels, using quantum methods for v=0-7 andquasi-classical methods up to v=12. Owing to the high energy barrier, theserate coefficients increase with v and also with the gas temperature. Weimplemented the new rates in the Meudon PDR code and studied their effect onmodels with different ultraviolet (UV) illumination conditions. In stronglyUV-irradiated dense gas (Orion Bar conditions), the presence of H2 in highlyvibrationally excited levels (v>7) enhances the NH abundance by two orders ofmagnitude (at the PDR surface) compared to models that use the thermal ratecoefficient for reaction N(4S) + H2 -> NH + H. The increase in NH columndensity across the PDR is a factor of ~25. We explore existing Herschel/HIFIobservations of the Orion Bar and Horsehead PDRs. We report a 3-sigma emissionfeature at the ~974 GHz frequency of the NH N_J=1_2-0_1 line toward the Bar.The emission level implies N(NH)~10^13 cm^-2, which is consistent with PDRmodels using the new rate coefficients for reactions between N and UV-pumpedH2. This formation route dominates over hydrogenation reactions involving theless abundant N+ ion. JWST observations will soon quantify the amount andreactivity of UV-pumped H2 in many interstellar and circumstellar environments.

The role of highly vibrationally excited H2 initiating the N chemistry: Quantum study and 3-sigma detection of NH emission in the Orion Bar PDR