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

Gas--grain astrochemical models predict that a substantial fraction of elemental nitrogen in dense cores and protoplanetary disks is locked in molecular nitrogen (N 2), in the gas or ice phase, although interstellar N 2 ice has not been directly identified. Rosetta measurements at comet 67P/Churyumov--Gerasimenko showed strong N 2 depletion relative to CO, while NH 3 was the dominant detected nitrogen-bearing ice. We experimentally quantify the conversion of NH 3 ice into N 2 and N 2 H 4 under astronomically relevant conditions by studying its temperature- and fluence-dependent VUV photochemistry. Experiments were performed under ultra-high vacuum using 40-monolayer NH 3 ice at 15, 25, and 70 K irradiated at 115--170 nm to a total fluence of 3.8 times10 18 photons cm-2. Laser desorption with post-ionization reflection time-of-flight mass spectrometry (ReTOF-MS) was used to monitor N 2 and N 2 H 4 simultaneously. VUV photolysis efficiently forms both products, with formation kinetics and product ratios depending on temperature and photon fluence. Initial-growth fits gave the highest apparent ice-retained N 2 formation yield at 15 K, approximately an order of magnitude above those at 25 and 70 K. The N 2 /NH 3 column-density ratio was determined as a function of fluence and compared with astronomical constraints. These results suggest that NH 3 ice photochemistry may provide an additional pathway to N 2 ice in cold outer planetary environments.