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.

Same as Fig. 2 for the OH –J = 1/2    →    3/2 cross-ladder lines at  ~79 μm, the OH J = 5/2    →    3/2 ground-state lines at  ~119 μm, and the OH J = 3/2    →    1/2 excited lines at  ~163 μm. Note the decrease of OH line intensity with distance from the ionization front.