P. Beiersdorfer

Laboratory Measurements and Modeling of the Fe XVII X-Ray Spectrum

Detailed measurements, line identifications, and modeling calculations of the Fe XVII L-shell emission spectrum between 9.8 and 17.5 A are presented. The measurements were carried out on an electron beam ion trap under precisely controlled conditions where electron-impact excitation followed by radiative cascades is the dominant line formation process. In addition to the strong transitions emanating from the n = 3 shell, we identify and accurately determine wavelengths for transitions from higher shells up to n = 11, including two electric quadrupole transitions that have not been previously identified. Various theoretical values, including new distorted wave calculations, are compared to our measurements, which establish definitive values for testing spectral modeling predictions. We find a value of 3.04 ± 0.12 for the ratio of the intensity of the 2p-3d1P1 resonance and of the 2p-3d3D1 intercombination line situated at 15.01 and 15.26 A, respectively. This value is higher than the values observed in solar spectra, which supports claims that the solar value is affected by resonant scattering. However, because our value is significantly lower than calculated values, the amount of scattering has probably been overestimated in past analyses. Comparisons of the measured intensity ratios of the transitions originating in levels of higher principal quantum number n with present distorted wave calculations show good agreement up to n = 6. The combined flux of all 2p-nd transitions with n ≥ 5 and all 2s-np transitions with n = 4 and 5 relative to the flux of the 15.01 A resonance line has been measured to be 0.13+ 0.04−0.03 .

The use of an electron beam ion trap in the study of highly charged ions

The Electron Beam Ion Trap (EBIT) is a relatively new tool for the study of highly charged ions. Its development has led to a variety of new experimental opportunities; measurements have been performed with EBITs using techniques impossible with conventional ion sources or storage rings. In this paper, I will highlight the various experimental techniques we have developed and the results we have obtained using the EBIT and higher-energy Super-EBIT built at the Lawrence Livermore National Laboratory.

Emission-Line Spectra of Ar IX-Ar XVI in the Soft X-Ray Region 20-50 Å

As part of a larger project to complete a comprehensive catalog of astrophysically relevant emission lines in support of new-generation X-ray observatories using the Lawrence Livermore electron beam ion traps EBIT-I and EBIT-II, we present observations of argon lines in the extreme-ultraviolet region. Our database includes wavelength measurements with standard errors, relative intensities, and line assignments for Ar IX-Ar XVI between 20 and 50 A. The experimental data are complemented with a full set of calculations using the Hebrew University Lawrence Livermore Atomic Code (HULLAC). Despite differences in calculated and measured wavelengths, we find the calculated lines to be of great utility in analyzing our laboratory spectra. The calculated line intensities are generally sufficient to identify the strongest transitions in each charge state. We note, however, an underestimation by theory of the strength of the 3s → 2p lines relative to the 3d → 2p lines in Ar IX, Ar X, and Ar XI. The laboratory data are compared with Chandra observations of Procyon, resulting in the identification of an Ar IX line that was previously thought to be from S IX.

The Electron‐Beam Ion Trap

The mention of few‐electron atoms usually brings to mind hydrogen, helium or other light elements in neutral form. However, these simple atoms are part of a sequence of ions having the same number of electrons but different nuclear charges. For example, the hydrogen‐like sequence spans neutral hydrogen through hydrogen‐like uranium, U91+. Both the atomic physics and the applications of the most highly charged ions in such isoelectronic sequences are receiving increasing attention. Recently the electronbeam ion trap has made it possible to produce and study any such ion in a modest‐sized apparatus (figure 1).

The magnetic trapping mode of an electron beam ion trap: New opportunities for highly charged ion research

Using x‐ray spectroscopic techniques, we have investigated the properties of an electron beam ion trap (EBIT) after the electron beam is switched off. In the absence of the electron beam, bare, and hydrogenlike Kr35+ and Kr36+ ions remain trapped due to externally applied magnetic and electric fields for at least 5 s; xenon ions with an open L shell, i.e., Xe45+–Xe52+, remain trapped at least as long as 20 s. The ion storage time in this ‘‘magnetic trapping mode’’ depends on the pressure of background atoms as well as on the value of the externally applied trapping potential, and even longer ion storage times appear possible. The magnetic trapping mode enables a variety of new opportunities for atomic physics research involving highly charged ions, which include the study of charge transfer reactions, Doppler‐shift‐free measurements of the Lamb shift, measurements of radiative lifetimes of long‐lived metastable levels, or ion‐ion collision studies, by x‐ray or laser spectroscopy, and mass spectrometry. Be...

High‐resolution x‐ray spectrometer for an electron beam ion trap

A high‐throughput von Hamos‐type Bragg crystal spectrometer is described that is operated with the Livermore electron beam ion trap. The spectrometer is employed to measure high‐resolution x‐ray spectra from highly charged heliumlike and neonlike ions. Data from heliumlike Ti20+ and Fe24+ and from neonlike Au69+ are presented to demonstrate the utility of the new instrument.

LABORATORY MEASUREMENTS AND IDENTIFICATION OF THE Fe xviii-xxiv L-SHELL X-RAY LINE EMISSION

A comprehensive survey of Dn � 1 iron L-shell X-ray line emission is presented. We have used the electron beam ion trap EBIT-II at the Lawrence Livermore National Laboratory and equipped with flat crystal spectrometers to measure the wavelengths of over 150 features between 10.6 and 18 A ˚ . We present wavelengths, line identifications, and relative intensities of all of the significant Dn � 1 L-shell line emission from Fe xviii– xxiv at electron densities of � 10 12 cm � 3 resulting from direct electron impact excitation from the ground state followed by radiative cascades. This data set includes 2–3 times as many lines as are found in current standard line lists. Subject headings: atomic data — line: identification — stars: coronae — Sun: corona — Sun: X-rays, gamma rays — X-rays: general

Experimental M1 Transition Rates of Coronal Lines from Ar X, Ar XIV, and Ar XV

Transition probabilities of three magnetic dipole (M1) transitions in multiply charged ions of Ar have been measured using the Livermore electron-beam ion trap. Two of the transitions are in the ground con—gurations of Ar XIV (B-like) and Ar IX (F-like), and are associated with the coronal lines at 4412.4 and 5533.4 respectively. The third is in the excited 2s2p con—guration of Be-like Ar XV and produces Ae , the coronal line at 5943.73 Our results for the three atomic level lifetimes are 9.32 ^ 0.12 ms for the Ae . Ar X 2s22p5 level, 9.70 ^ 0.15 ms for the Ar XIV 2s22p level, and 15.0 ^ 0.8 ms for the Ar XV 2P 1@2 2P 3@2 2s2p level. These results diUer signi—cantly from earlier measurements and are the most accurate 3P 2 ones to date. Subject headings: atomic datamethods: laboratory

Diagnostic Utility of the Relative Intensity of 3C to 3D in Fe XVII

The relative intensity R of the resonance and intercombination line in neon-like Fe XVII, located at 15.01 and 15.26 A, respectively, has been measured at the Lawrence Livermore National Laboratory electron beam ion trap EBIT-II as a function of the relative abundance of sodium-like Fe XVI. Our measurements identify several Fe XVI lines and one Fe XV line in this region. We show that an Fe XVI inner shell satellite line coincides with the intercombination line and can significantly reduce the apparent R. We measure R = 1.90 ± 0.11 when the relative abundance of Fe XVI to Fe XVII is ~1. This explains the anomalously low ratios observed in the solar and stellar coronae. The fact that the apparent relative intensity of the resonance and intercombination line in Fe XVII is sensitive to the strength of an Fe XVI inner shell satellite, and therefore, the relative abundance of Fe XVI to Fe XVII, makes the line ratio a diagnostic of temperature.

An unexpectedly low oscillator strength as the origin of the Fe xvii emission problem

Highly charged iron (Fe16+, here referred to as Fe xvii) produces some of the brightest X-ray emission lines from hot astrophysical objects, including galaxy clusters and stellar coronae, and it dominates the emission of the Sun at wavelengths near 15 ångströms. The Fe xvii spectrum is, however, poorly fitted by even the best astrophysical models. A particular problem has been that the intensity of the strongest Fe xvii line is generally weaker than predicted. This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions, fuelling a continuing controversy over whether this discrepancy is caused by incomplete modelling of the plasma environment in these objects or by shortcomings in the treatment of the underlying atomic physics. Here we report the results of an experiment in which a target of iron ions was induced to fluoresce by subjecting it to femtosecond X-ray pulses from a free-electron laser; our aim was to isolate a key aspect of the quantum mechanical description of the line emission. Surprisingly, we find a relative oscillator strength that is unexpectedly low, differing by 3.6σ from the best quantum mechanical calculations. Our measurements suggest that the poor agreement is rooted in the quality of the underlying atomic wavefunctions rather than in insufficient modelling of collisional processes.