Measured time evolutions of the normalized numbers (n_x(t)/n_Σ0) of the primary NH ions and of the produced NH ions. The measurements were performed at the collisional temperature T = 39 K, hydrogen number density [H₂]=6.2 × 10¹⁰ cm⁻³, and helium number density [He]∼10¹⁴ cm⁻³. The solid lines are fits of the measured data. The crosses (x) indicate the sum of the normalized numbers of all ions in the ion trap, Σ(t).