Ok. I think I can see a way to resolve this. Although in a 'free-free' state, a photon will "see" and respond with a proton-electron pair (a hydrogen plasma is visulaized here) if that electron is within the Debye length of the proton. This length decreases with decreasing temperature as we...
I am trying to form some intuitive concept about the free-free absorption coefficient for a hydrogen plasma.
The standard expression (cf. e.g. Schwarzschild's Structure and Evolution of the Stars, 1958, p64) contains a reasonably familiar particle type cross-section component, but there is also...