Polarization of the Lyman line emission following radiative recombination of bare ions: effects of relativity and radiation multipoles

We have calculated the degree of linear polarization of the Ly-α1 (2p3/2 → 1s1/2) and Ly-β1 (3p3/2 → 1s1/2) lines emitted after radiative recombination of spinless bare nuclei with unidirectional, unpolarized electrons. These calculations have been performed for various recombined H-like ions with atomic numbers in the range 10 ≤ Z ≤ 92. Several incident-electron kinetic energies e0 ranging from 0.01 to 10 times the electron binding energy in the 1s1/2 ground state were considered and the radiative cascades following recombination into states with principal quantum number up to n = 6 were included. Three sets of cross sections for radiative recombination to individual magnetic sublevels of the excited levels of the recombined H-like ions were computed: the first two in the electric-dipole approximation with the nonrelativistic and relativistic electron wavefunctions and the third one in the exact relativistic method including all multipole orders of the radiation field. Results for the polarization calculated using these three sets of cross-section data were compared in order to stress the relative importance of relativistic and multipole effects as Z or e0 increases.