Rotational energy diagram of ¹⁶OH showing the lines detected by Herschel/PACS (in microns). Detected transitions are shown in black while other pumping transitions discussed in the text are shown in red. OH and rotational ladders are produced by the spin-orbit interaction, while the Λ-doubling splitting of each rotational level is produced by the nuclei rotation and the unpaired electron motion. The Λ-doubling splitting has been enhanced for clarity. Hyperfine structure is not shown.