Quantifying the temperature-dependent yields of N2 and N2H4 formation in vacuum-ultraviolet-irradiated NH3 ice
Abstract
Gas--grain astrochemical models predict that a substantial fraction of elemental nitrogen in dense cores and protoplanetary disks is locked in molecular nitrogen (N2), in the gas or ice phase, although interstellar N2 ice has not been directly identified. Rosetta measurements at comet 67P/Churyumov--Gerasimenko showed strong N2 depletion relative to CO, while NH3 was the dominant detected nitrogen-bearing ice. We experimentally quantify the conversion of NH3 ice into N2 and N2H4 under astronomically relevant conditions by studying its temperature- and fluence-dependent VUV photochemistry. Experiments were performed under ultra-high vacuum using 40-monolayer NH3 ice at 15, 25, and 70 K irradiated at 115--170 nm to a total fluence of 3.8×1018 photons cm−2. Laser desorption with post-ionization reflection time-of-flight mass spectrometry (ReTOF-MS) was used to monitor N2 and N2H4 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 N2 formation yield at 15 K, approximately an order of magnitude above those at 25 and 70 K. The N2/NH3 column-density ratio was determined as a function of fluence and compared with astronomical constraints. These results suggest that NH3 ice photochemistry may provide an additional pathway to N2 ice in cold outer planetary environments.
Versions
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BibTeX
@misc{samarth2026quantifying,
title = {{Quantifying the temperature-dependent yields of N$_2$ and N$_2$H$_4$ formation in vacuum-ultraviolet-irradiated NH$_3$ ice}},
author = {P. Samarth and G. Fedoseev and S. Ioppolo and L. Hornekær and E. F. van Dishoeck and K. -J. Chuang},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.17878},
eprint = {2609.17878},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.17878},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Quantifying the temperature-dependent yields of N_2 and N_2H_4 formation in vacuum-ultraviolet-irradiated NH_3 ice
AU - P. Samarth
AU - G. Fedoseev
AU - S. Ioppolo
AU - L. Hornekær
AU - E. F. van Dishoeck
AU - K. -J. Chuang
PY - 2026
DA - 2026/09/15
PB - arXiv
DO - 10.48550/arxiv.2609.17878
UR - https://arxiv.org/abs/2609.17878
AB - 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_2H_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 3.8×10^18 photons cm^−2. Laser desorption with post-ionization reflection time-of-flight mass spectrometry (ReTOF-MS) was used to monitor N_2 and N_2H_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.
ER -