We have measured the absolute polarization of the 2p 6 1 S 0 Ϫ2p 5 ( 2 P 3/2 o )3s͓3/2͔ 2 o magnetic quadrupole transition in Ne-like barium, excited in an electron-beam ion trap at a variety of energies. We find strong evidence for the existence of resonant excitation processes that are not explained by our collisional-radiative calculations even when the polarization arising from impact excitation is included. At energies well away from where the resonances occur, the agreement between experiment and theory is good.
The polarisation of line radiation emitted during dielectronic recombination is investigated theoretically. By taking into account interference effects due to overlapping levels, formulae have been derived for the linear polarisation of these lines (usually called dielectronic satellites). Special attention is given to recombination of He-like systems which play an important role in hot plasmas. As an illustration, for n = 4 , results are reported for Li-like Is'nl-ls2pnl satellites which contribute to the observed intensity of the He-like ls2-ls2p parent line. The interference effects are found to increase the polarisation up to a factor of two as n + m. Expressions are also given for the polarisation of well isolated Li-like satellite lines excited by an arbitrary axially symmetric electron angular distribution. It is shown that detectable ( 2 1 0 °/ ~) polarisation of j and k satellites may be expected in a low-density plasma provided the dominant thermal electrons contribute by less than about 70% of the total emission of these satellites. Finally, the combined effects of anisotropy and power-law energy distribution of a suprathermal electron tail on the intensity ratio j/w are examined. It is concluded that to neglect the presence of a small percentage of these suprathermal electrons in a thermal plasma may lead to strong overestimates of the plasma temperature.
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