Ammonia deuteration mechanism in interstellar conditions

Aims. We investigate the reaction mechanism of deuterium transfer in the D3+ + NH3 reaction. The distinction between proton hop and full scrambling mechanisms is critical for accurate modeling of deuterated ammonia isotopologs. Methods. The reaction rate coefficients and product branching fractions were measured in a cryogenic 22-pole radiofrequency ion trap between 110 and 300 K. The D3+ ions were stored in helium buffer gas at number densities of 1013−1014 cm−3 and reacted with ammonia at controlled number densities of 1010−1011 cm−3. Results. The reaction proceeds with reaction rate coefficients decreasing from (5.4 ± 1.1) × 10−9 cm3s−1 at 149 K to (3.9 ± 0.8) × 10−9 cm3s−1 at 293 K, close to the capture rate coefficient predicted by the average dipole orientation theory. The measured reaction rate coefficients exhibit a systematic isotope effect compared to the analogous H3+ + NH3 system, with D3+ reacting slightly more slowly. The product branching fractions reveal that NH3D+ is the dominant product throughout the temperature range, with a branching fraction greater than 99.5% at low temperatures. This observation confirms that the reaction proceeds via the proton hop mechanism rather than full scrambling.