Evolution of the N₂D⁺/N₂H⁺ ratio with time for the slow (right) and fast (left) models as a function of CO depletion (depletion multiplied by 2 in the top row, X[CO] = 1  ×  10^-6, divided by 5 in the bottom row, X[CO] = 1  ×  10^-5). The dashed lines trace the reference results (from respectively the slow and fast cases displayed in Fig. 5). The dotted line traces the new versus reference model ratio for the last epoch (corresponding to a peak density of 2  ×  10⁶ cm^-3) in each frame. The results are traced for the same density profiles as those in Fig. 2.