Effects of initial-state population variations on the 2p→1s Kαdielectronic satellite spectra of highly ionized iron ions in high-temperature astrophysical and laboratory plasmas

Abstract

Theoretical predictions are presented for the iron Ea x-ray emission spectra from hightemperature plasmas, assuming steady-state optically thin excitation conditions. Account has been taken of all fine-structure components of the 2p~1 s inner-shell-electron radiative transitions in the iron ions from Fe xvIII to Fe xxrv. The Ka emission spectra are assumed to be produced by means of dielectronic recombination and inner-shell-electron collisional excitation processes that involve intermediate autoionizing states belonging to electronic configurations of the type 1s'2s'2p'. In addition to the electron-temperature variation, which is attributable to the temperature dependences of the radiationless electron capture and inner-shell-electron collisional excitation rate coefficients and to the temperature dependence of the charge-state distribution, the Ka emission spectra exhibit an electron-density sensitivity. This electron-density sensitivity is a result of the density-dependent distribution of populations among the different fine-structure levels of the initial ions in the dielectronic recombination and inner-shell electron collisional excitation processes. In order to introduce a simplified treatment for the initial distribution of populations, whose precise determination would involve the detailed and self-consistent description of a multitude of elementary atomic autoionization, collision, and radiation processes, the electron-density range of interest has been subdivided into three, increasingly dense, regions. In the low-density region, which is expected to be appropriate for astrophysical plasmas such as solar flares and supernova remnants, it has been assumed that only the lowest-lying fine-structure levels of the ground-state electronic configurations of the initial ions are populated. Magnetically confined laboratory plasmas, such as tokamaks, are represented by the intermediate-densityregion, in which the initial ion populations have been assumed to be statistically distributed among all fine-structure levels of the ground-state electronic configurations. In the high-density region, which is expected to occur in laser-produced and vacuum-spark-produced plasmas, the populations of the initial ions have been assumed to be statistically distributed among all fine-structure levels of not only the ground-state electronic configurations but also the additional configurations which can be derived from the ground-state configurations by means of 2s~2p excitations. The inclusion of these additional excited configurations of the initial ions not only alters the intensities of the satellite lines that are predominant at low densities, but it also introduces additional satellite lines that occur at different wavelengths. Discussions are presented on the consequences of this electron-density sensitivity of the Ka satellite emission for the spectroscopic determinations of electron temperatures, electron densities, and charge-state distributions in both astrophysical and laboratory plasmas.

A complete set of radiative and Auger rates for K-vacancy states in Fe XVIII–Fe XXV

“…In the case of the 219 transitions listed in HFR4, wavelengths are even shorter: ∆λ(HFR4, AST3) = −2.2 ± 0.9 mÅ. This indicates that Jacobs et al (1989) computed wavelengths with ab initio level energies. COR, SAF and MCDF respectively give values for 73, 101 and 497 transitions in ions with 4 ≤ N ≤ 6.…”

“…CI is only assumed within the n = 2 complex. HFR4: Wavelengths, A-values, satellite intensity Q d factors, and radiative branching ratios, B r , calculated for Fe -Fe  by Jacobs et al (1989) with . This external data set includes only the dominant transitions listed in Table III and IV of this paper.…”

“…A survey of the radiative and autoionization data for Fe ions with electron occupancies N ≤ 6 computed by Chen (1986), Chen & Crasemann (1987, 1988, Chen et al (1997) and those contained in the "Cornille" and "Safronova" data sets compiled by Kato et al (1997) results in notable discrepancies that undermine confidence in spectral modeling. Moreover, with the exception of the extensive study by Jacobs et al (1989), there has hardly been any attention in ions with N > 6. In the present work, A-values for the allowed and forbidden valence-valence transitions within the n = 2 complex have also been treated because the post-Auger radiative signature, rich in UV and optical emission lines, has been shown to have astrophysical potential (Shapiro & Bahcall 1981).…”

Atomic X-ray spectroscopy of accreting black holes

“…We have added one further simplification, which is to ignore possible collisional redistribution of levels u, which is likely to be a valid assumption for most astrophysical densities. The above derivation shows that a fluorescence yield cannot be considered as an intrinsic attribute of an atomic species, a fact that has been known for some time [195]. Equation (134) shows that Y K depends on the level population distribution of i − 1, which depends in turn on the electron density, the electron temperature, and the local radiation field.…”

Iron Kα Spectra from an Atomic Modeling Perspective

“…Underlying the mechanism is collisional excitation from ground to low-lying meta-stable levels, which, when photoionized out of the Is shell, leads to fluorescence lines that are distinct from those produced in the low-density limit (Jacobs et al 1989). The critical electron densities are ~ 10 13 c m -3 (Liedahl et al 1992).…”