Schematic of the excitation of OH in the presence of prompt emission. The production of rotationally excited OH (orange arrows) is followed by a N → N – 1 radiative cascade (shown as blue arrows) emitting the series of lines shown in Fig. 3. For lower rotational states (N ≲ 10), OH levels can be excited by IR pumping and collisions that emit in the FIR. In this greatly simplified schematic, only a limited number of rotational levels are shown, ignoring the splitting of rotational levels caused by Λ-doubling and spin-orbit coupling, vibrational levels, and other kinds of weaker radiative transitions.