Model results for H₃O⁺ and NH₃ in Arp 220. a) Ratios of pure inversion metastable lines of H₃O⁺ versus T_rot : K = 6/K = 4 (red), K = 8/K = 4 (blue), K = 9/K = 4 (green), and K = 10/K = 4 (black). b) For NH₃, the fluxes of the (red), (green), and (blue) are normalized to the flux of the 124.9 μm feature (composed of the 4₀−3₀ and NH₃ lines, Paper I). c) Ratios of rotation-inversion lines of H₃O⁺ as a function of T_dust . Dashed lines in all panels indicate the measured values, with typical uncertainties of 20%.