Energy level diagram of OH showing the transitions detected in Mrk 231 with Herschel/PACS and Spizer/IRS with blue and green arrows, respectively. The l-doubling splitting of the levels is not indicated. Colored numbers indicate rounded wavelengths in μm. The Π_1/2  J = 5/2−3/2 (black line) and the Π_1/2−Π_3/2  J = 3/2−5/2 doublets at λ_rest ≈ 99 and 96 μm (the latter detected in NGC 4418, G-A12) were not observed, as they are redshifted into the PACS gap at 100 μm. In the text, we denote a doublet by giving its wavelength (e.g. OH119).