POLARIZATION OF FE XXV 1S(2)-1S2L LINES - COLLISIONAL RESONANCES AND RADIATIVE CASCADE CONTRIBUTIONS TO 1S2L MAGNETIC-SUBLEVEL EXCITATION RATES

The effects of collisional resonances and radiative cascades on the linear polarization of He-like iron (Fe XXV) lines from 1s2l to 1s2 levels have been investigated. Detailed calculations have been carried out for the 1s3l3l' resonance contributions to electron-impact excitation rates from the 1s' ground level to the individual magnetic sublevels of 1s2l configurations. Excitation collision strengths from 1s2 to 1s3l sublevels as well as radiative cascade transitions from 1s3l to 1s2l' sublevels have also been computed. The autoionization transition-matrix elements (for the resonance effects) and the collison strengths (for the radiative cascades) have been computed in the distorted-wave approximation using intermediate coupling with fine-structure mixing multiconfiguration bound wave functions. The results indicate that the collisional resonance contributions, when averaged over a small energy range just covering them, have a somewhat significant depolarizing effect on the (1s2 1S0 - 1s2p 1P1, electric dipole) w line, the (1s2 1S0 - 1s2p 3P2, magnetic quadrupole) x line, and the (1s2 1S0 - 1s2p 3P1, fine-structure electric dipole) y line. However, the averaged polarization degree over some single resonances can reach high values. Now for the (1s2 1S0 - 1s2S 3S1 , relativistic magnetic dipole) z line the individual resonances are found to induce a polarizaton degree less than 15% in absolute value but after averaging over all 1s3l3l' resonances z remains practically unpolarized. Concerning radiative cascades, it is shown that they can create a relative small degree of polarization on the z line, the highest value being close to 14%. For w, x, and y lines, the cascades have a weak depolarizing effect, except for x at large incident energy. It is expected that the contributions from 1snl cascades as well as 1s3lnl' resonances, with n greater-than-or-equal-to 4, would not greatly change the results already obtained.