2013ApJ...774..105B

Cosmic rays are possibly the main agents to prevent the freeze-out of molecules onto grain surfaces in cold dense clouds. Ammonia (NH₃) is one of the most abundant molecules present in dust ice mantles, with a concentration of up to 15% relative to water (H₂O). FTIR spectroscopy is used to monitor pure NH₃ and NH₃-H₂O ice samples as they are irradiated with Ni and Zn ion beams (500-600 MeV) at GANIL/France. New species, such as hydrazine (N₂H₄), diazene (N₂H₂isomers), molecular hydrogen (H₂), and nitrogen (N₂) were identified after irradiation of pure NH₃ices. Nitrous oxide (N₂O), nitrogen oxide (NO), nitrogen dioxide (NO₂), and hydroxylamine (NH₂OH) are some of the products of the NH₃-H₂ O ice radiolysis. The spectral band at 6.85 µm was observed after irradiation of both types of ice. Besides the likely contribution of ammonium (NH₄⁺) and amino (NH₂) radicals, data suggest a small contribution of NH₂ OH to this band profile after high fluences of irradiation of NH₃-H₂ O ices. The spectral shift of the NH₃"umbrella" mode (9.3 µm) band is parameterized as a function of NH₃/H₂ O ratio in amorphous ices. Ammonia and water destruction cross-sections are obtained, as well as the rate of NH₃-H₂ O (1:10) ice compaction, measured by the OH dangling bond destruction cross-section. Ammonia destruction is enhanced in the presence of H₂ O in the ice and a power law relationship between stopping power and NH₃ destruction cross-section is verified. Such results may provide relevant information for the evolution of molecular species in dense molecular clouds.