Normalized photon intensity of the OH(X²Π)(v = 0) intra-ladder mid-IR lines as a function of the quantum number of the upper energy level N_up. The photon intensities are computed for the fiducial values of the parameters and divided by Φ (see Eq. (13)). Circle and triangle markers correspond to lines belonging to the Ω = 1∕2 and Ω = 3∕2 ladders, respectively. Λ-doublets are indiscernible in this plot. In this regime, rotational levels are only populated by the radiative cascade of OH photofragments and , depends only on the spectral shape of the adopted radiation field. The solid line is an analytical model that assumes that the intra-ladder lines within the v = 0 state are powered by the radiative cascade of the OH fragments produced in the v = 0 state. The dotted line is an analytical model that assumes that any OH produced in a vibrational state instantaneously decays toward the ground vibrational state with negligible change in its rotational number (see Appendix C).