Continuum-normalized spectra around the H₃O⁺ lines in NGC 4418 (upper histograms) and Arp 220 (lower histograms). The dotted vertical lines indicate the rest wavelengths of the transitions for the nominal redshifts of z = 0.00705 (NGC 4418) and 0.0181 (Arp 220). Panels a) − i) show the pure inversion, metastable lines of H₃O⁺, while panels j) − p) show the rotation-inversion lines at λ < 100 μm. The spectra in panels h) and j) − p) have been scaled by a factor 2 and 3, respectively. The and lines at ≈ 181 μm are strongly contaminated by lines of CH and HO, and are not shown. Close features due to other species and potential contaminations are indicated. In panels f) and i), the contamination by NH₂, HCN, and OH⁺ is expected to be small (see text). The blue lines show the best-fit model for H₃O⁺; the green lines in panels g) and i) show model fits for NH₃ and NH lines; red is the total.