Quantifying the temperature-dependent yields of N 2 and N 2 H 4 formation in vacuum-ultraviolet-irradiated NH 3 ice

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Context. Gas–grain astrochemical models commonly predict that a substantial fraction of elemental nitrogen in dense cores and protoplanetary disks is locked up in molecular nitrogen (N 2 ), either in the gas or ice phase. However, the direct identification of interstellar N 2 ice is still lacking. Measurements by ESA’s Rosetta space mission at comet 67P/Churyumov–Gerasimenko revealed a high depletion of N 2 with respect to CO, while indicating that NH 3 ice is the dominant nitrogen reservoir among the detected species. The origin of the low N 2 cometary ice abundance and relation with NH 3 ice is still poorly constrained. Aims. This experimental study aims to quantify the conversion of NH 3 ice into N 2 and N 2 H 4 under various astronomically relevant conditions. With this goal, we systematically investigated how the photochemistry of NH 3 ice depends on temperature and photon fluence, which are relevant to star-forming regions. Methods. Experiments were performed in an ultra-high-vacuum setup to study the kinetics of the vacuum-ultraviolet (VUV) photon irradiation (115−170 nm) of 40 monolayer-thick NH 3 ice at 15, 25, and 70 K up to a total photon fluence of 3.8 × 10 18 photons cm −2 . Laser desorption with post-ionization reflection time-of-flight mass spectrometry (ReTOF-MS) was applied to monitor N 2 and N 2 H 4 formation simultaneously. Results. VUV photolysis of NH 3 ice results in the efficient formation of N 2 and N 2 H 4 , and the formation kinetics and photoproduct ratios depend on temperature and applied photon fluence. The corresponding formation yields were derived from linear fits to the initial product growth, using fluence ranges of < 1 × 10 18 photons cm −2 for N 2 and < 2 × 10 18 photons cm −2 for N 2 H 4 . The highest apparent ice-retained N 2 formation yield was observed at 15 K, exceeding those at 25 and 70 K by approximately an order of magnitude. The ratio of N 2 to NH 3 column densities was determined as a function of fluence and compared with astronomical constraints. The experiments suggest that NH 3 ice photochemistry could represent an additional formation pathway for N 2 ice in cold outer planetary environments.