Formation of Nitrogen and Hydrogen‐bearing Molecules in Solid Ammonia and Implications for Solar System and Interstellar Ices

Abstract

We irradiated solid ammonia (NH 3) in the temperature range of 10Y60 K with high-energy electrons to simulate the processing of ammonia-bearing ices in the interstellar medium and in the solar system. By monitoring the newly formed molecules online and in situ, the synthesis of hydrazine (N 2 H 4), diazene (N 2 H 2 isomers), hydrogen azide (HN 3), the amino radical (NH 2), molecular hydrogen (H 2), and molecular nitrogen (N 2) has been confirmed. Our results show that the production rates of hydrazine, diazene, hydrogen azide, molecular hydrogen, and molecular nitrogen are higher in amorphous ammonia than those in crystalline ammonia; this behavior is similar to the production of molecular hydrogen, molecular oxygen, and hydrogen peroxide found in electron-irradiated water ices. However, the formation of hydrazine in crystalline ammonia does not show any temperature dependence. Our experimental results give hints to the origin of molecular nitrogen in the Saturnian system and possibly in the atmospheres of proto-Earth and Titan; our research may also guide the search of hitherto unobserved nitrogen-bearing molecules in the interstellar medium and in our solar system.

On the Formation of Amide Polymers via Carbonyl–amino Group Linkages in Energetically Processed Ices of Astrophysical Relevance

“…The second largest ion peak is observed at a mass-to-charge ratio of m/z = 32 and is attributable to hydrazine (N 2 H 4 , IE = 8.1 eV, m/z = 32). This trace has an onset temperature of 170 K and peaks at 210 K, to decrease down to almost zero intensity at 300 K. The assignment is based on comparisons with earlier results obtained on electron irradiated ammonia (NH 3 ) ices (Zheng et al 2008;Förstel et al 2015b). Despite its low ionization energy, almost no hydrazine signal is detected in the 9.0 eV experiment, because the ionization cross-section of hydrazine at 9.0 eV is a factor of more than 100 lower than at 10.49 eV (Syage et al 1992).…”

Formation of Hydroxylamine (NH2OH) in Electron-Irradiated Ammonia−Water Ices

“…25 This radical has also been observed in our laboratory in electron-irradiated pure ammonia ices. 15 The absorption features of the newly formed hydroxylamine emerged only when the samples were warmed up to about 174 K, that is, at a temperature when most water and ammonia sublimed. Figure 4 shows the decay of these features during the warm-up of the postirradiation sample.…”

“…15 Also, the unimolecular decomposition of a water (H 2 O) was found to produce a hydroxyl (OH) plus a hydrogen atom (reaction 2). 17 The hydrogen atoms can recombine to form molecular hydrogen via reaction 3.…”

“…17 Similarly, hydrazine (NH 2 NH 2 ) can be formed via the combination of two NH 2 radicals (reaction 5); this process was also monitored in electronirradiated neat ammonia samples at 10 K. 15 Hydroxylamine (NH 2 OH) is likely synthesized through the reaction between an amino radical and a hydroxyl radical in an exothermic reaction (reaction 6, ∆H 0 r,298 ) -278 kJ mol -1 ).…”

“…12,13 Very recently, laboratory experiments implied that the nitrogen in Saturn's ring and the water vapor plume on Enceladus might result from the irradiation of an ammonia-water ice mixture on the surface of Enceladus. 14,15 Irradiation exposures of ammonia-water ice have also been investigated with 0.8 MeV protons by Moore et al 16 at 10 and 80 K. However, chemically correct formation routes and the temperature-dependent yields of the detected molecules have remained highly speculative and elusive. These limited studies on the underlying chemistry of ammonia-water ices at different temperatures call for a systematic experimental study utilizing both infrared spectroscopy and mass spectrometry simultaneously to give detailed and complementary information on the underlying chemical and physical processes during the irradiation of ammonia-water ices.…”