= Photoionization-recombination thermal balance
When the number of <photoionizations> equals the number of recombinations, and only those processes exchange <thermal energy>, <equilibrium> requires $\langle h\nu-h\nu_0\rangle_{\rm abs}=\langle E_{\rm rec}(T)\rangle$. <Photon> absorption and <electron> capture have their own spectral weights. The density cancels from this event balance, but the actual <equilibrium> temperature requires the two energy averages.<Heating and Cooling of HII regions by Ian Czekala>[Photoionization heating and Recombination radiation sections]{ref}
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