J. L. Schwob

Bio: J. L. Schwob is an academic researcher from The Racah Institute of Physics. The author has contributed to research in topics: Ionization & Spectral line. The author has an hindex of 18, co-authored 58 publications receiving 1174 citations.

The 1s-3p Kβ-like x-ray spectrum of highly ionized iron

Abstract: Measurements of the fine structure on the high-energy side of the Fe Kβ x-ray line, obtained from low-inductance high-current vacuum sparks, are reported. Fully relativistic central field calculations for atoms, including magnetic Breit interactions and estimates of the Lamb shifts, correlate the experimental features to the 1s-3p transition of the various Fe ions. The features are shown to be emitted by ions from Fe ix to Fe xviii, and Fe xxiv to Fe xxvi.

Line and Band Emission from Tungsten Ions with Charge 21+ to 45+ in the 45-79 Å Range

Abstract: We have measured the radiation from medium-charge-state ions of tungsten in the spectral range from 45 to 70 \AA{}. The ions were produced in an electron beam ion trap (EBIT), and the radiation was observed using a 2-m grazing-incidence spectrometer. Operating EBIT with beam energies between 500 eV and 4 keV allowed us to sample charge states ranging from I-like ${\mathrm{W}}^{21+}$ to Cu-like ${\mathrm{W}}^{45+}.$ Lines of Sr-like through Cu-like tungsten were observed; for the Sr-like to As-like ions new lines were identified. Analysis of the spectra is based on ab initio calculations using the relativistic HULLAC code. For charge states lower than Sr-like ${W}^{36+},$ the spectrum lines fuse into a bright emission band situated around $50 \mathrm{\AA{}}.$ The band extends over an interval of approximately $2 \mathrm{\AA{}}\mathrm{}$ and moves smoothly towards shorter wavelengths with decreasing ion-charge state. The shift and narrowing of the band emission cannot be interpreted in the framework of the standard unresolved transition array formalism assuming statistical population of the excited levels. Instead, the observations are explained by detailed calculations based on a collisional-radiative model. It is shown that the variation of the experimental spectrum with charge state is the signature of the low electron density in EBIT.

Interpretation of the quasicontinuum band emitted by highly ionized rare-earth elements in the 70-100-Å range.

Abstract: Narrow (\ensuremath{\Delta}\ensuremath{\lambda}5 \AA{}), intense quasicontinuum bands, appearing in emission spectra of highly ionized rare-earth elements between 70 and 100 \AA{}, previously observed in tokamaks and laser-produced plasmas, have been obtained from a low-inductance vacuum spark. The bands shift toward shorter wavelengths with increasing atomic number Z. Using the unresolved transition array model, these bands are identified as primarily 4d-4f transitions in Rh i to Rb i--like ions, although the widths come out too large and the mean wavelengths are much too dependent on ionization stages. Detailed ab initio computations show that the interactions between the 4${p}^{6}$4${d}^{N\mathrm{\ensuremath{-}}1}$4f and 4${p}^{5}$4${d}^{N+1}$ configurations are responsible for the narrowing and the superposition of the transition arrays for the different ionization stages of a given element, in agreement with experimental data.

Collisional Plasma Models with APEC/APED: Emission-Line Diagnostics of Hydrogen-like and Helium-like Ions

Abstract: New X-ray observatories (Chandra and XMM-Newton) are providing a wealth of high-resolution X-ray spectra in which hydrogen- and helium-like ions are usually strong features. We present results from a new collisional-radiative plasma code, the Astrophysical Plasma Emission Code (APEC), which uses atomic data in the companion Astrophysical Plasma Emission Database (APED) to calculate spectral models for hot plasmas. APED contains the requisite atomic data such as collisional and radiative rates, recombination cross sections, dielectronic recombination rates, and satellite line wavelengths. We compare the APEC results to other plasma codes for hydrogen- and helium-like diagnostics and test the sensitivity of our results to the number of levels included in the models. We find that dielectronic recombination with hydrogen-like ions into high (n = 6-10) principal quantum numbers affects some helium-like line ratios from low-lying (n = 2) transitions.

The flexible atomic code

Abstract: We describe a complete software package for the computation of various atomic data such as energy levels; radiative transition; collisional excitation; ionization by electron impact, photoionizatio...

Updated Atomic Data and Calculations for X-Ray Spectroscopy

Abstract: We describe the latest release of AtomDB, version 2.0.2, a database of atomic data and a plasma modeling code with a focus on X-ray astronomy. This release includes several major updates to the fundamental atomic structure and process data held within AtomDB, incorporating new ionization balance data, state-selective recombination data, and updated collisional excitation data for many ions, including the iron L-shell ions from Fe+16 to Fe+23 and all of the hydrogen- and helium-like sequences. We also describe some of the effects that these changes have on calculated emission and diagnostic line ratios, such as changes in the temperature implied by the He-like G-ratios of up to a factor of two.

HULLAC, an integrated computer package for atomic processes in plasmas

Abstract: We describe H ULLAC , an integrated code for calculating atomic structure and cross sections for collisional and radiative atomic processes. This code evolved and has been used over the years, but so far, there was no coherent, comprehensive, and in-depth presentation of it. It is based on relativistic quantum mechanical calculations including configuration interaction. The collisional cross sections are calculated in the distorted wave approximation. The theory and code are presented, emphasizing the various novel methods that has been developed to obtain accurate results very efficiently. In particular we describe the parametric potential method used for both bound and free orbitals, the factorization–interpolation method applied in the derivation of collisional rates, the phase amplitude approach for calculating the continuum orbitals and the N JGRAF graphical method used in the calculation of the angular momentum part of the matrix elements. Special effort has been made to insure the simplicity of use, which is demonstrated in an example.

Indirect X-Ray Line-Formation Processes in Iron L-Shell Ions

Abstract: We present a systematic study of the role of indirect processes in the soft X-ray line formation of iron L-shell ions, using a newly developed, relativistic, multiconfigurational atomic package. These indirect processes involve the neighboring charge states of the target ion, namely, radiative recombination, dielectronic recombination, and resonance excitation. For Fe XVII, the inner-shell collisional ionization (CI) is also relevant. Lines originating from 3s and 3p upper levels of Fe XVII-XX are found to be significantly affected by these processes, with some lines being enhanced by nearly a factor of 2 at the temperature of maximum fractional abundance in CI equilibrium. Such enhancement, although not enough to completely explain the observed line ratios from various astrophysical sources, is a vast improvement over the previous models neglecting these processes. Rate coefficients for individual processes are tabulated, which can be conveniently included in spectral models to correctly account for these effects. As a by-product of this investigation, the total recombination and ionization rates of all Fe L-shell ions are tabulated as a function of temperature.