Effect of inner-shell ionization on the circular polarization of the Fe24+ (1s2s)1→(1s2)0 line produced by collisions with a longitudinally polarized electron beam

We have theoretically studied the influence of K-shell ionization of Li-like Fe on the circular polarization of the (1s2s) 1 →(1s 2 ) 0 or 1s2s 3 S 1 →1s 2 1 S 0 forbidden line of the heliumlike Fe 2 4 + ion, when the incident electron beam is assumed to be longitudinally polarized. The required cross sections for ionization to the ( 1s2s) 1 M j =-1, 0, and + I magnetic sublevels were calculated in a relativistic distorted-wave approximation for incident electron energies up to 2000 Ry. New cross sections were used for excitation of the Fe 2 4 + ion from the ground level to each magnetic sublevel of the upper line level as well as the higher cascading levels. These were computed in a relativistic distorted-wave method that includes the generalized Breit interaction between the bound and free electrons. Our calculations show that for a relatively large range of impact energies above the ionization threshold, the inner-shell ionization populates more preferentially the M j =+ I sublevel relative to the M j =- I sublevel than does the combination of direct collisional excitation and radiative cascades, and its inclusion results in an increase of the circular polarization degree of the line, depending on the Li- to He-like abundance ratio. However, at higher impact energies, the inner-shell ionization has the effect of decreasing the degree of circular polarization.