Measured time evolutions of the normalized numbers (n_x(t)/n_Σ0) of indicated ions in the trap. A⁺(15 Da) indicates a normalized number of ions with mass 15 Da (mixture of ¹⁴NH⁺ and ¹⁵N⁺ ions). The measurements were performed at T = 34 K, [H₂]=6.2 × 10¹⁰ cm⁻³, and [He]≈10¹⁴ cm⁻³. The solid lines are least-squares fits, which were used to determine the reaction rate coefficients. The dash-dotted straight line indicates the decrease in the normalized numbers of ¹⁴NH⁺ ions and the dotted line indicates the normalized numbers of ¹⁵N⁺ ions. The crosses (×) indicate the sum of the normalized numbers of all ions in the ion trap, Σ(t). The normalized numbers of ions are increased by a factor of 2.