Parametrization of Absorption-Line Profiles

Abstract

The parametrization $\sigma \mathrm{}\left(E\right)=C+\Sigma \stackrel{}{K}\frac{\left[\right(\frac{1}{2}{\Gamma }_K)B_K+(E-E_K\left)A_K\right]}{[{(E-E_K)}^2+{\left(\frac{1}{2}{\Gamma }_K\right)}^2]}$ 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.