N + H surface reaction under interstellar conditions: Does the NH/NH2/NH3 distribution depend on N/H ratio?

Abstract

Astronomical observations have already reported that the abundances of NH, NH2, and NH3 cannot be explained with gas phase chemistry modeling and depend strongly on the environment where these three N-hydrides have been detected. Consequently, dust grains may play a crucial role in the formation of these species through surface reactions. We have investigated the formation of NH, NH2 and NH3 through N→+HNH→+HNH2→+HNH3 solid state reaction at 3 and 10 K, in different environments. The N2–H2 gas binary mixture is then plasma activated and the resulting gas from the microwave discharge made of H, H2, N, N2 and NH3 is deposited onto the surface of the sample holder maintained at 3 and 10 K. We show that, while at 10 K, NH, NH2 and NH3 have been detected in different abundance distributions, at 3 K the NH and NH2 radicals have not observed and the low amount of NH3 present in our solid samples comes from the N2–H2 plasma discharge. This is the first experimental study showing all the steps leading to ammonia formation, by detecting the final reaction product NH3 and the two reaction intermediates NH and NH2. In addition to support the idea of NH and NH2 formation on interstellar dust grains, we show that the environment where the reaction takes place plays an important role in the formation and stabilization of these two reactive unsaturated N-hydride radicals. We emphasize that NH and NH2 are detected only if there are enough nitrogen molecules in the solid samples to isolate them from reacting, otherwise, directly exposed to H-atoms they turn into stable saturated species such as NH3 and NH3-aggregates.