Density dependence of the polarization of the 23P1 → 11S0 intercombination line emitted by helium-like neon excited by an electron beam

The linear polarization of the x-ray 1s2p(3)P(1) -> 1S(2) S-1(0) intercombination line emitted by helium-like Ne8+ ion collisionally excited by a monoenergetic electron beam has been theoretically studied versus electron densities, from 10(9) to 5 x 10(13) cm(-3). In addition to direct excitation from the 1s(2) ground level and radiative cascades from 1s3l higher levels, excitation from the 1s2s S-3(1) metastable level was included in the collisional-radiative model. Collision strengths for transitions between magnetic sublevels were computed at three incident electron energies above the 1s(2) -> 1s2l excitation threshold (68, 90 and 140 Ry, i.e. 925, 1225 and 1905 eV), using a semi-relativistic distorted-wave approximation for partial waves up to l = 40 and a Coulomb-Bethe top-up for higher l. For electron densities below 10(10) cm(-3), the intercombination line is found to be polarized between -49.1% and -41.0% in the range of impact energies chosen. With increasing density, the polarization degree rapidly decreases in absolute value to zero, then becomes positive. Similar results for the polarization of the 1s2S(3)S(1) -> 1S(2) S-1(0) forbidden and 1s2p(3)P(2) -> 1s2S(3)S(1) dipole-allowed lines are also given.