Energy level diagrams of H₂O and H₂O⁺ shown in black and red, respectively. Dark blue arrows are the submm H₂O transitions we have observed in this work. Pink dashed lines show the far-infrared pumping path of the H₂O excitation in the model we use, with the wavelength of the photon labeled. The light blue dashed arrow is the transition from para-H₂O energy level 2₂₀ to 2₁₁ along the cascade path from 2₂₀ to 1₁₁. Rotational energy levels of H₂O and H₂O⁺, as well as fine structure component levels of H₂O⁺  are also shown in the figure.