High-frequency photoabsorption by an ion immersed in a plasma as calculated from Bloch’s hydrodynamic model

The high-frequency behavior of the photoabsorption cross section of an ion immersed in a plasma is investigated in the framework of the Thomas–Fermi model, supplemented with Bloch’s hydrodynamic equations. It is shown that the singular nature of the electron density profile near the point nucleus, derived in Thomas–Fermi approximation, causes enhanced high-frequency absorption proportional to the inverse square of frequency. According to an exact quantummechanical relation the electron density profile is not singular. It is shown that in the hydrodynamic model a regular density profile leads to a photoabsorption cross section which decays as the inverse sixth power of frequency. Thus for a regular density profile collective effects described in the hydrodynamic model rapidly become less important at high frequency. In this regime the photoabsorption cross section is dominated by single electron inverse bremsstrahlung.