Diffuse continuum radiation is emitted by three processes: thermal bremsstrahlung, radiative recombination, and two-photon decays of metastable levels. The thermal bremsstrahlung emissivity is given by [Osterbrock 1974]:

where T is the electron temperature, is the electron abundance, Z is the charge on the most abundant ion, is the abundance of that ion, and is a Gaunt factor ([Karzas and Latter, 1966]). For two photon decays, we adopt the distribution ([Tucker and Koren 1971]):

where is the excitation energy.

Tim Kallman 2017-08-08