Radiative Recombination Continuum

Radiative recombination occurs when an electron collides and recombines with an ion, emitting a photon in the process. It is one of two primary methods of recombination, the other being dielectronic recombination. In radiative recombination, the energy of this photon will equal the kinetic energy of the electron plus the binding energy of the newly-recombined electron. Since the kinetic energy of the electron is not quantized, this forms a continuous spectrum with sharp edges at the binding energy of the levels. The spectrum of this radiation is called the "Radiative Recombination Continuum", or RRC for short.

The cross section for radiative recombination is related via a detailed balancing argument to the photoionization cross section, using a method first derived by Milne (1924, Phil. Mag., 47, 209). As a favor to graduate students taking Radiative Processes classes, we include a complete derivation in this memo on the radiative recombination continuum. (Many thanks to Yuanzhen Han (Purple Mountain Observatory, Chinese Academy of Sciences) for correcting some errors in the original version of the memo.)

In a hot, optically-thin thermal plasma, the radiative recombination continuum is normally a perturbation on top of the bremsstrahlung continuum. However, at low (kT~0.1 keV) temperatures, the bremsstrahlung contribution is quite small and the RRC due to recombining oxygen ions (primarily O+6 and O+7) is large. Therefore, at low energies, the abundance of oxygen becomes a parameter that can affect the entire continuum, not merely the emission lines.

In a hot, optically-thin thermal plasma that is out of ionization equilibrium, the RRC features may be heightened or removed, depending upon the circumstances of the plasma. An ionizing plasma (ie, one recently heated by a shock) will show very weak RRC emission, since there are relatively few highly-ionized ions that are able to recombine. In contrast, a recombining plasma (such as could occur when a hot plasma is rapidly cooled by expansion) will show very strong RRC features, since the majority of the ions are recombining.