Rate of H⁻ photodetachment by Lyα cooling radiation (Eq. (7)), for the same calculations shown in Figs. 1 and 2. The photodetachment rate is slightly higher for lower levels of the background radiation J₂₁ due to the temperature dependence of the cross section for Lyman α scattering. At low densities the gas evolves adiabatically, leading to the scaling R_detach∝n^3/2, whereas at higher densities the scaling is better approximated assuming the gas is isothermal, leading to the scaling R_detach∝n^31/18. Note that, in all cases, the photodetachment rate rises above the critical rate of ~10^-5 s^-1 found in calculations assuming a constant background rate and a weak H₂-dissociating radiation field by Agarwal et al. (2016a).