Parametrization of Absorption-Line Profiles

The parametrization ${σ}(E)=C+{Σ}{}{K}{[(1/2{{Γ}}K){B}K+(E{-}{E}K){A}K]}{[{(E{-}{E}K)}²+{(1/2{{Γ}}K)}²]}$ for attenuation cross sections (particularly photo-ionization) is discussed, with attention to the following details: prescriptions for exact calculation of profile parameters, in which the effects of distortions are separated from the effects of multiple scatterings; the validity of assuming independent (noninterfering) resonances; the specific case of autoionizing lines; connections with alternative parametrizations; the prohibition on negative cross sections assured by unitarity; behavior at threshold; and the applicability of this parametrization to emission lines.