 Free-free absorption

Free-free absorption is the inverse of bremsstrahlung: a free electron absorbs a photon during an encounter with an ion, which permits conservation of momentum. In local thermodynamic equilibrium its absorption coefficient and the thermal free-free emissivity obey Kirchhoff's law of thermal radiation, $j_{\nu }={\alpha }_{\nu }{B}_{\nu }(T)$. For a nonrelativistic plasma at $h\nu \ll k_{B}T$, ${\alpha }_{\nu }\propto n_e{\sum }_i{Z}_i^2{n}_i{T}^{-3/2}{\nu }^{-2}{g}_{\mathrm{ff}}$, including the stimulated-emission correction in the net coefficient.