Predicted OH⁺ abundance (upper panel), OH⁺/H₂O⁺ and OH⁺/H₃O⁺ ratios (mid panels), and H⁺ and e⁻ abundances (lower panel), as a function of the molecular fraction f_H2 (up to approximately , see Eq. (2)). Each curve corresponds to a value of ζ/n_H, indicated in the upper panel in units of cm³ s^-1. Solid and dashed curves correspond to T_gas = 150 and 550 K, respectively. The inferred values of χ_OH⁺, R₁, and R₂, in the nuclear regions of NGC 4418 and Arp 220, and in the C_extended of Arp 220, are indicated with the shaded dark and light regions, respectively. In the lower panel, squares and triangles show the predicted χ_H⁺ and χ_e.