Evolution of molecular ions abundances in the shock as a function of time (“STEP 2”). The colours indicate HCO⁺ (black), N₂H⁺(blue), HOCO⁺ (red), SO⁺ (green), and HCS⁺ (magenta) abundances. Observed abundances (colour vertical bars) are overplotted at the shock age (t_shock ~ 2000 years, vertical dotted line). The evolution is computed from steady-state values in the cloud (n_H2= 10⁴ cm^-3, T_kin = 10 K, ζ = 3 × 10^-16 s^-1) by enhancing the gas temperature and density (n_H2= 10⁵ cm^-3, T_kin = 70 K), and the abundance of molecules which are thought to be sputtered off dust grain mantles: CO₂, S, H₂S, and OCS (panels C), D), E), and F), respectively). When varying S, H₂S, and OCS abundances, X_CO2 is set to = 6 × 10^-5. Finally, panel G) shows the evolution of molecular ions abundances in our “best-fit model”, i.e. for X_CO2= 6 × 10^-5 and X_OCS= 6 × 10^-6.