= 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 $h\nu\ll k_BT$, $\alpha_\nu\propto n_e\sum_i Z_i^2n_i T^{-3/2}\nu^{-2}g_{\rm ff}$, including the stimulated-emission correction in the net coefficient.
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