Effects of the radiative recombination on the intensity and polarization of the Ly-α emission of hydrogen-like ions

Abstract

The intensity ratio of the Ly - α 1 ( 2 p 3 / 2 → 1 s 1 / 2 ) to Ly - α 2 ( 2 p 1 / 2 → 1 s 1 / 2 ) photon emission is analyzed for hydrogen-like Fe 25 + ions if their excitation arises in a plasma not only from the electron impact but also due to the radiative recombination (RR) of initially bare Fe 26 + ions. Under such conditions, the intensity ratio and the (degree of) linear polarization of the Ly - α 1 line are explored for collisions with an electron beam over a wide range of kinetic energies up to 50 keV. Apart from the direct population of the 2 p 1 / 2 , 3 / 2 levels via the RR of bare ions, the contributions from radiative cascades and higher multipoles are taken into account by applying a fully relativistic theory for the motion of the electrons and the electron–photon coupling. Our calculations show an overall small effect of the RR upon the degree of the Ly - α 1 polarization as well as the Ly - α 1 / Ly - α 2 intensity ratio under usual plasma conditions. However, the effects from the RR of initially bare ions may become significant at electron beam energies ∼ 7 – 10 keV , for plasma conditions far away from ionization equilibrium with a relatively large proportion of bare Fe ions, as it may be realized in electron-beam ion trap experiments.