Collisional-radiative calculations for theJ= 0−1 lasing line of neon-like germanium under anisotropic excitation conditions

Abstract

A new asymmetry parameter characterizing the differences between the polarized π and σ gain components of the soft-x-ray J=0-1 lasing line of neon-like ions is calculated in the case of Ge 22+ assuming an electron distribution which is a weighted sum of an isotropic Maxwellian and a monoenergetic beam. Using a quasi steady-state collisional-radiative model, we determine in the weak amplification regime the relative populations of the upper M=0 and lower M 0, 1 =  magnetic sublevels of the lasing line as a function of electron density from 10 20 to 2 10 21 ´cm −3 . This model includes inelastic and elastic collisional transitions, as well as spontaneous radiative decay between all the 337 M-sublevels arising from the 75 lowest-lying Ge 22+ J-levels. The computations were performed for a temperature T e of the Maxwellian component between 1.2 10 6 ´and 8 10 6 ´K, a kinetic energy E 0 and a fraction f of the beam component in the ranges 1.5 20 keV -and 0.1% 10% -, respectively. The basic atomic data, such as level energies, radiative decay probabilities and inelastic collision strengths, were calculated with the flexible atomic code. However, some modifications of this code were made to get the collision strengths for transitions between M-sublevels due to impact with isotropic electrons as well as due to impact with an electron beam in the case of de-excitation. We find that the newly introduced asymmetry parameter may become significant under certain conditions of electron distribution corresponding to relatively low T e (1.2 10 2.5 10 6 6´-K) and E 0 (3-6 keV). The results reported here may be useful in the evaluation of the polarization degree of the J=0-1 x-ray laser output from a germanium plasma in the presence of fast directional electrons.

Local time dependence of the magnetic sublevel population and polarization of fluorescence radiation

“…Finally, we note that the recent calculations of Rachedi et al [9] and Bentotoche et al [7] were made assuming steady state, i.e. that the emission spectra are from ions in which the final-level populations are reached very quickly, so that no time dependence is present.…”

“…Over the past several decades, extensive investigations have been carried out in order to understand the patterns of fluorescence radiation after the electron-impact excitation (EIE) of highly charged ions [7][8][9][10][11]. The first measurement of the polarization was made in the experiment on the detection of photons emitted from transitions in He-like Sc 19+ ion by using an electron beam ion trap (EBIT) at two electron-beam energies of 4.36 and 5.62 keV [12].…”

Dominance of the density effects in the magnetic sublevel population and circular polarization of x-ray radiation

“…The third term describes the contribution to the 2 P M J f population arising from cascades from higher magnetic sublevels D J M k k k . A and C are, respectively, the transition rate (s −1 ) and the collisional excitation rate coefficient (cm 3 s −1 )[32][33][34][35]…”