7.2 Free-free radiation
The blackbody spectrum is emitted when thermally emitting matter is optically thick. Optically thin matter can also emit thermal radiation. Whenever a charged particle is accelerated it emits electromagnetic radiation. When the acceleration is due to the electric field of another charged particle the emitted radiation is called free-free emission or bremsstrahlung. (Bremsstrahlung is a German word meaning ‘braking radiation’.) The radiation emitted by an optically thin, thermal equilibrium distribution of electrons is called thermal bremsstrahlung, which is a bit of a mouthful, so astronomers sometimes colloquially refer to it as ‘thermal brems’.
The ‘free-free’ label makes an analogy with the formation of atomic spectral lines. When an electron in an atom makes a transition to another atomic energy level this is a bound-bound transition, because the electron is bound to the atom in both the initial and final states. Photoionization of an atom is a bound-free transition, because the electron is removed from the atom into a ‘free’ state. Conversely recombination of an ion and an electron, which liberates recombination radiation is a free-bound transition. The energy levels in an atom are discrete, so a bound-bound transition results in a spectral line at the precisely defined wavelength corresponding to the transition in question. There are ‘free’ quantum states at essentially all energies above the ionization energy, hence free-free emission, produced by free electrons in the Coulomb fields of ions, is a continuum spectrum.