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M. F. Gu, S. M. Kahn, D. W. Savin, E. Behar (Phys. Dept. and Columbia Astrophyics Laboratory), P. Beiersdorfer, G. V. Brown, D. A. Liedahl, K. J. Reed (Dept. of Phys. and Space Tech. LLNL.)
The unprecedented spectral resolution of the newly launched satellite observatories, \textit{Chandra} and \textit{XMM-Newton}, is capable of revealing very complicated discrete spectroscopic features in the entire soft X-ray band (0.1 -- 10 keV). Among these features, line emission from iron L-shell ions, Fe XVII through Fe XXIV at ~ 1 keV is of particular importance due to the high emissivity and rich diagnostic potential of these charge states. Quantitative analyses of such spectroscopic data require a thorough understanding of the physical processes responsible for the line emission. Using the electron beam ion trap at Lawrence Livermore National Laboratory, we investigate experimentally various line formation processes that are responsible for iron L-shell line emission. The cross sections of these processes are measured for relatively strong 3\to 2 and 4\to 2 lines in Fe XXI -- XXIV over a range of collision energies extending from well below the direct excitation thresholds to ~ 4 times the ionization potential of the ions.
The comparison with theoretical results shows that the distorted wave approximation is reasonably accurate in the energy range free of resonant processes. For the transitions investigated in our experiment, resonant excitation is shown to have minor effects on the line emission rate coefficients for thermal plasmas in collisional ionization equilibrium. The dielectronic recombination satellites can , however, enhance the rate coefficients by ~ 5 -- 15% at temperatures where the ion abundance peaks in collisional ionization equilibrium. For plasmas that have ionization temperatures much larger than electron kinetic temperatures, these resonant processes play an increasingly important role.