Photoabsorption by an ion immersed in a plasma at any temperature

The photoabsorption cross-section of an ion immersed in a plasma is studied on the basis of the Thomas-Fermi approximation for the equilibrium electro n distribution and Bloch's classical hydrodynamic model for collective moti on of the electrons. The frequency-dependent cross-section scales with the nuclear charge, and depends strongly on the plasma density and temperature. An approximation of the frequency dependence is constructed with the aid o f sum rules and Pade approximants.