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Ortho-Para Selection Rules in the Gas-phase Chemistry of Interstellar Ammonia

Abstract : The ortho-para chemistry of ammonia in the cold interstellar medium is investigated using a gas-phase chemical network. Branching ratios for the primary reaction chain involved in the formation and destruction of ortho- and para-NH3 were derived using angular momentum rules based on the conservation of the nuclear spin. We show that the "anomalous" ortho-to-para ratio of ammonia (~0.7) observed in various interstellar regions is in fact consistent with nuclear spin selection rules in a para-enriched H2 gas. This ratio is found to be independent of temperature in the range 5-30 K. We also predict an ortho-to-para ratio of ~2.3 for NH2. We conclude that a low ortho-to-para ratio of H2 naturally drives the ortho-to-para ratios of nitrogen hydrides below the statistical values.
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https://hal-insu.archives-ouvertes.fr/insu-03617070
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Submitted on : Wednesday, March 23, 2022 - 11:07:19 AM
Last modification on : Sunday, June 26, 2022 - 12:18:20 PM

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A. Faure, P. Hily-Blant, R. Le Gal, C. Rist, G. Pineau Des Forêts. Ortho-Para Selection Rules in the Gas-phase Chemistry of Interstellar Ammonia. The Astrophysical Journal Letters, 2013, 770, ⟨10.1088/2041-8205/770/1/L2⟩. ⟨insu-03617070⟩

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