Quantifying the temperature-dependent yields of N$_2$ and N$_2$H$_4$ formation in vacuum-ultraviolet-irradiated NH$_3$ ice

RESUMO

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.