Density dependence of intensity ratios of the 3d → 2p x-ray anisotropic emission from electron-beam excited Ar¹³⁺ ions

Abstract

We investigate the ratio R of the intensity of the blended lines 3d3/2,5/2 → 2p3/2 over that of the resonance line 3d3/2 → 2p1/2 emitted by boron-like Ar13+ ions after impact excitation by an electron beam. We calculate R as a function of the electron density ne in the range 109–1013 cm−3, for incident-electron energies from 0.46 to 5.5 keV. The calculations take into account the anisotropy in the angular distribution of photon emission by considering several angles of observation θ relative to the electron beam direction. A detailed collisional-radiative model that involves 560 magnetic sublevels arising from the 12 configurations 2s22p, 2s2p2, 2p3, 2s23l, 2s2p3l and 2p23l (l = s, p, d) is used to determine the populations of upper sublevels of the lines. All basic atomic parameters needed in the calculations, including radiative transition probabilities and collision strengths for transitions between magnetic sublevels, are obtained from the flexible atomic code. The ratio R at the angle θ = 0° is found to be higher than that at θ = 90° by up to 43% for ne = 109 cm−3, but this difference drops with increasing density to become less than 10% above ∼8 × 1011 cm−3. The line 3d3/2 → 2p3/2 blended with 3d5/2 → 2p3/2 is found to enhance the anisotropy of R. The present calculations may be extended to non-Maxwellian hot plasmas having cylindrically symmetric electron velocity distributions, which would be useful for electron density diagnostics of such plasmas.