Nuclear spin effects in ammonia chemistry

Ion trap study of NH₃⁺ and NH₄⁺ formation

Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180 00 Prague, Czech Republic

^★ Corresponding author: stepan.roucka@mff.cuni.cz

Received: 10 October 2025
Accepted: 3 November 2025

Abstract

Aims. We investigate whether the nuclear-spin state of H₂ (ortho or para state) modifies the kinetics of two key reactions in ammonia formation in the interstellar medium: NH₂⁺ + H₂ → NH₃⁺ + H and NH₃⁺ + H₂ → NH₄⁺ + H.

Methods. The reaction rate coefficients were measured in a cryogenic 22-pole radiofrequency ion trap over 15-270 K using normal H₂ (ortho-to-para ratio 3 : 1) and para-enriched H₂ (99.5% para-H₂) produced by catalytic conversion at ≈15 K.

Results. For NH₂⁺ + H₂, the reaction rate coefficients are in the range 10⁻¹⁰-10⁻⁹ cm³ s⁻¹ with a weak negative temperature dependence. For NH₃⁺ + H₂, the reaction rate coefficients exhibit an Arrhenius dependence with an activation energy of ≈90 meV at high temperatures (>100 K), combined with an increasing value towards low temperatures (10-100 K) due to quantum tunneling. In the studied range of temperatures, the reaction rate coefficients obtained with para-enriched and normal H₂ are indistinguishable within the experimental uncertainty for both reactions.