V. Mäckel

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.

EXPERIMENTAL INVESTIGATIONS OF ION CHARGE DISTRIBUTIONS, EFFECTIVE ELECTRON DENSITIES, AND ELECTRON-ION CLOUD OVERLAP IN ELECTRON BEAM ION TRAP PLASMA USING EXTREME-ULTRAVIOLET SPECTROSCOPY

Spectra in the extreme ultraviolet range from 107 to 353 A emitted from Fe ions in various ionization stages have been observed at the Heidelberg electron beam ion trap (EBIT) with a flat-field grating spectrometer. A series of transition lines and their intensities have been analyzed and compared with collisional-radiative simulations. The present collisional-radiative model reproduces well the relative line intensities and facilitates line identification of ions produced in the EBIT. The polarization effect on the line intensities resulting from nonthermal unidirectional electron impact was explored and found to be significant (up to 24%) for a few transition lines. Based upon the observed line intensities, relative charge state distributions (CSD) of ions were determined, which peaked at Fe23+ tailing toward lower charge states. Another simulation on ion charge distributions including the ionization and electron capture processes generated CSDs which are in general agreement with the measurements. By observing intensity ratios of specific lines from levels collisionally populated directly from the ground state and those starting from the metastable levels of Fe XXI, Fe X and other ionic states, the effective electron densities were extracted and found to depend on the ionic charge. Furthermore, it was found that the overlap of the ion cloud with the electron beam estimated from the effective electron densities strongly depends on the charge state of the ion considered, i.e. under the same EBIT conditions, higher charge ions show less expansion in the radial direction.

Photoionization of N3 + and Ar8 + in an electron beam ion trap by synchrotron radiation

Photoionization (PI) of multiply and highly charged ions was studied using an electron beam ion trap and synchrotron radiation at the BESSY II electron storage ring. The versatile new method introduced here extends the range of ions accessible for PI investigations beyond current limitations by providing a dense target of ions in arbitrary, i.e. both low and high charge states. Data on near-threshold PI of N3 + and Ar8 + ions, species of astrophysical and fundamental interest, show high resolution and accuracy allowing various theoretical models to be distinguished, and highlight shortcomings of available PI calculations. We compare our experimental data with our new fully relativistic PI calculations within a multiconfiguration Dirac?Fock approach and with other advanced calculations and find generally good agreement; however, detailed examination reveals significant deviations, especially at the threshold region of Ar8 +.

X-ray laser spectroscopy of highly charged ions at FLASH

Laser spectroscopy, widely applied in physics and chemistry, is extended into the soft x-ray region for the first time. Resonant fluorescence excitation of highly charged ions (HCIs) by soft x-ray free-electron lasers (FELs) shows here the potential for unprecedented precision on photonic transitions hitherto out of reach. The novel experiments combine an electron beam ion trap (EBIT) with the Free-electron LASer at Hamburg (FLASH) to measure resonant fluorescence by trapped HCIs as a function of the lasers wavelength. The present experiments have already reached the performance of conventional soft and hard x-ray spectroscopy. We present the results obtained for three fundamental and theoretically challenging transitions in Li-like ions, namely 1s22s?2S1/2?1s22p?2P1/2 in Fe23+ at 48.6 eV, in Cu26+ at 55.2 eV and 1s22s?2S1/2?1s22p?2P3/2 in Fe23+ at 65.3 eV. The latter demonstrates laser spectroscopy of multiply or HCIs at more than one order of magnitude higher energies than hitherto reported. Resolving power leading to relative precision up to 6 parts-per-million points to the possibility of providing an atomic absolute wavelength standard in this spectral region, which is still lacking.

Coronium in the Laboratory: Measuring the Fe XIV Green Coronal Line by Laser Spectroscopy

The green coronal line at 530.3 nm was first observed during the total solar eclipse of 1869. Once identified as emitted by Fe XIV, it became clear that this highly charged ion was typical for the range of temperatures found in coronal plasmas, stellar winds, outflows, and accretion disks. Under these conditions of high ionization, the strongest transitions are in the X-ray, extreme ultraviolet, and ultraviolet wavelength range, with only few optical lines. For these so-called forbidden coronal lines, only scarce laboratory data is available, and even advanced atomic theory codes cannot yet predict their wavelengths with the accuracy required for precise absolute velocity determinations of such plasmas. Here we report on a study of the Fe XIV line, a key coronal transition of a highly charged ion, using laser spectroscopy in an electron beam ion trap, obtaining the first laboratory measurement of 530.2801(4) nm for its rest wavelength. The result enables the determination of absolute line shifts and line broadenings in hot turbulent plasmas and astrophysical environments, with an error bar of only 0.24 km s–1. In addition, our measurement provides a much-needed benchmark for advanced atomic structure calculations, which are fundamental for astronomy.

On the Transition Rate of the Fe x Red Coronal Line

We present a lifetime measurement of the 3s 23p 5 2 Po 1/2 first excited fine-structure level of the ground state configuration in chlorine-like Fe X, which relaxes to the ground state through a magnetic dipole (M1) transition (the so-called red coronal line) with a wavelength accurately determined to 637.454(1) nm. Moreover, the Zeeman splitting of line was observed. The lifetime of 14.2(2) ms is the most precise one measured in the red wavelength region and agrees well with advanced theoretical predictions and an empirically scaled interpolation based on experimental values from the same isoelectronic sequence.

Photoionization of ions in arbitrary charge states by synchrotron radiation in an electron beam ion trap

Photoionization of ions in various charge states is studied with an electron beam ion trap at the synchrotron BESSY II. The ion target density achieved by this method, representing an increase of up to four orders of magnitude with respect to conventional techniques, gives unprecedented access to photoionization of highly charged ions at photon energies reaching the keV range. Data on near-threshold photoionization of N3+, Ar12+, Fe12+ combined with measurements on neutral gas targets in the same setup demonstrate the versatility of this technique and show both very good resolution and accuracy.

Laser spectroscopy of highly charged argon at the Heidelberg electron beam ion trap

We report on two-level laser spectroscopy on the electron dipole-forbidden 1s22s22p 2P3/2?2P1/2 transition in boron-like Ar13+ ions stored in an electron beam ion trap. By monitoring the laser-induced fluorescence as a function of the laser frequency, the transition wavelength was determined to be 441.25575(17)?nm. The accuracy achieved in this first investigation is equal to that of the best wavelength measurements in highly charged ions.

Studies of highly charged iron ions using electron beam ion traps for interpreting astrophysical spectra

For over a decade, the x-ray astrophysics community has enjoyed a fruitful epoch of discovery largely as a result of the successful launch and operation of the high resolution, high sensitivity spectrometers on board the Chandra, XMM-Newton and Suzaku x-ray observatories. With the launch of the x-ray calorimeter spectrometer on the Astro-H x-ray observatory in 2014, the diagnostic power of high resolution spectroscopy will be extended to some of the hottest, largest and most exotic objects in our Universe. The diagnostic utility of these spectrometers is directly coupled to, and often limited by, our understanding of the x-ray production mechanisms associated with the highly charged ions present in the astrophysical source. To provide reliable benchmarks of theoretical calculations and to address specific problems facing the x-ray astrophysics community, electron beam ion traps have been used in laboratory astrophysics experiments to study the x-ray signatures of highly charged ions. A brief overview of the EBIT-I electron beam ion trap operated at Lawrence Livermore National Laboratory and the Max-Planck-Institut fur Kernphysiks FLASH-EBIT operated at third and fourth generation advanced light sources, including a discussion of some of the results are presented.

X-ray laser spectroscopy with an electron beam ion trap at the free electron laser LCLS

We present a first laser spectroscopy experiment in the keV energy regime, performed at the Free-Electron Laser LCLS at Stanford. An electron beam ion trap was used to provide a target of highly charged O, F and Fe ions. The resonant fluorescence spectra obtained for various transitions were calibrated to simultaneously measured Lyman lines of hydrogenic ions.