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

Abstract

The intensity ratio of the Ly - alpha(1) (2p(3/2) -> 1s(1/2)) to Ly - alpha(2) (2p(1/2) -> 1s(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 - alpha(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 2p(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 - alpha(1) polarization as well as the Ly - alpha(1)/Ly - alpha(2) intensity ratio under usual plasma conditions. However, the effects from the RR of initially bare ions may become significant at electron beam energies similar to 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.