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

Abstract

The linear polarization of the x-ray 1s2p3P1 → 1s21S0 intercombination line emitted by helium-like Ne8+ ion collisionally excited by a monoenergetic electron beam has been theoretically studied versus electron densities, from 109 to 5 × 1013 cm−3. In addition to direct excitation from the 1s2 ground level and radiative cascades from 1s3l higher levels, excitation from the 1s2s3S1 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 1s2 → 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 1010 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 1s2s3S1 → 1s21S0 forbidden and 1s2p3P2 → 1s2s3S1 dipole-allowed lines are also given.