CH⁺ line surface brightness variation with wavelength as predicted by RADEX considering the physical condition at ro-vibrational emission peak from the CLOUDY model noted in Table 3. The left panel shows this without chemical pumping, and the right panel considers chemical pumping. Upper panel: full spectrum in the wavelength range 0–200 μm. Lower panel: most intense ro-vibrational line spectrum in the wavelength range 3–5.5 μm.