In a fully ionized plasma, in which all bound electrons have been removed from their original atoms or molecules, there is besides the continuous spectrum no line emission or absorption, except possibly for features related to plasma resonances or waves (Bekefi 1966, Stix 1992, Swanson 1989). These often nonthermal features are usually at such low frequencies that they do not obscure or interfere with atomic radiation, the subject of principal interest here. Since atoms and incompletely stripped ions possess a continuous spectrum, besides the discrete spectrum providing the pairs of states involved in line radiation, continuous emission and absorption spectra underly and accompany the line spectra discussed in the preceding chapters. These processes are not only important as background to line emission, but also because continuum intensities can provide relatively direct measures of electron density and temperature (see sections 10.2 and 11.4, respectively).

With the usual convention for the energies of bound states as being negative relative to those of zero kinetic energy electrons at large distances from the nucleus of any isolated atom or ion, one might infer that all positive energy states belong to the continuous spectrum. In practice, this is an oversimplification, because there are states corresponding to the excitation of two or more bound electrons which are almost discrete.