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

P. Samarth; G. Fedoseev; S. Ioppolo; L. Hornekær; E. F. van Dishoeck; K. -J. Chuang

doi:10.1051/0004-6361/202658992

paper

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

arXiv:2609.17878 · doi:10.1051/0004-6361/202658992

Abstract

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 $3.8 \times 1 0^{18}$ 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.

22 pages, 7 figures

Quantifying the temperature-dependent yields of N2​ and N2​H4​ formation in vacuum-ultraviolet-irradiated NH3​ ice

Abstract

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