Optical Properties of Plasmas Based on an Average-Atom Model

Optical properties of plasmas, including dielectric constants, indices of refraction, and absorption coefficients, are determined from an average-atom point of view. Linear response of an average atom to an harmonic electric field leads to an averageatom version of the Kubo-Greenwood formula, which is used to calculate the frequencydependent electric conductivity of the plasma. The frequency-dependent dielectric function is determined from the conductivity using Kramers-Kronig dispersion relations. The index of refraction and absorption coefficient of the plasma are subsequently obtained from the dielectric function. Comparing the present results with the free-electron model helps one understand anomalies observed recently in space and time resolved interferograms of Al plasmas produced by 13.9 nm and 14.7 nm x-ray lasers.