Home
The experimental setup at FLASH: Lithium-like Fe23+ ions were produced in a new electron beam ion trap depicted at the top of the figure. The highly charged ions were excited by femtosecond soft X-ray FEL pulses (yellow). Since the lifetime of the excited state is just 0.55 nanoseconds the relaxation results in immediate emission of fluorescence radiation, which was collected by X-ray mirrors and focused onto a microchannel plate detector.
Soft X-ray Spectroscopy on trapped Fe23+
Highly charged ions are abundant in the universe, but virtually no experimental data exists on their interactions with energetic photons. FLASH and future FELs may determine the electronic structure of highly charged ions with unprecedented precision providing critical input to stellar models and enabling high-accuracy tests of the theory of quantum electrodynamics.
Highly charged ions stripped off most of their electrons, up to bare nuclei, constitute a dominant fraction of the visible matter in stars, supernovae, near-stellar clouds, and jets from active galactic nuclei; all abundant phenomena in the universe which astronomers have recently begun to explore with unprecedented accuracy in satellite missions like the XMM-Newton and the Chandra X-ray space telescope.
Precise knowledge of the electromagnetic line spectrum of highly charged ions is indispensable to understand these cosmic objects as well as the hot plasmas used in fusion energy research. However, up to date our insight into the interactions between highly charged ions and very energetic radiation, associated with e.g. supernovae and jets, is based almost entirely on theoretical model calculations. This is due to the fact that laser spectroscopy has been limited by the lack of appropriate light sources beyond the vacuum-ultraviolet spectral range. FLASH is the first in a series of upcoming FEL devices, like the LCLS in Stanford and the European XFEL in Hamburg, which will extend the accessible spectral range from a few eV to several keV. Thus, direct resonant laser spectroscopy experiments on highly charged ions, where one-electron bound-bound transitions up to 130 keV occur, will become feasible.
Testing QED Theory
Apart from their importance in astrophysics, the comparably simple electronic structure of highly charged ions makes them an ideal testing ground for the quantum theory of electromagnetic interactions, quantum electrodynamics (QED). This is the most precise theory in physics because no discrepancies between theory and experiment have been found up to now. QED is a quantum field theory of the electromagnetic force and is hence part of the standard model of particle physics. Experiments at FLASH and XFEL will allow for very stringent tests of theoretical QED predictions of the interactions between highly charged ions and very energetic radiation.
The few remaining electrons orbiting the atomic nucleus in such ions experience extreme electromagnetic fields, and field-dependent effects are strongly boosted by the powers of the nuclear charge (Z) determining the field strength. The binding energy scales with Z2, and the QED corrections of the binding energies, the Lamb shift, for example, already with Z4.
Due to the steep scaling with Z, tunable soft and hard X-ray FELs can probe fundamental field-dependent effects such as electronic transitions in highly charged ions. Thus, by measuring transition energies in different highly charged ions all the way up to U89+ QED theory can tested and refined.
Caught in the Trap
Highly charged ions up to Fe23+ have been produced in a new electron beam ion trap installed in the FLASH experimental hall. Successive electron impact ionization of the atoms by the electron beam of the trap strips off most of their electrons. The resulting highly charged positive ions are trapped by the negative space charge of the electron beam and by appropriate potentials of the trap electrodes. The trapped ions form a cylindrical cloud of 50 mm length and 200 - 300 μm diameters with a density of about 1010 ions/cm3.
In the experiments the ions in the trap were excited by intense femtosecond soft X-ray pulses from FLASH. The excited states live less than a nanosecond, and as the ions return to their ground state they emit fluorescence radiation, which is recorded by a detector. By using tunable, monochromatic FEL pulses the transition energies between the ground state and excited states in highly charged ions can be mapped out with unprecedented accuracy.
The photon energy range covered by FLASH and upcoming hard X-ray FELs will extend the accessible spectral range by several orders of magnitude (the scale is partly logarithmic). With the measurements of electronic transitions of Fe23+ at FLASH this region has now been made accessible (yellow ball). The transition energy for the 1s22s - 1s22p excitation of some lithium-like systems (Kr33+, Xe51+, etc.) is shown in green. For comparison the figure displays some benchmarking VUV laser spectroscopy experiments on electronic transitions in neutral atoms (blue balls) as well as the most advanced laser spectroscopy experiments on transitions in highly charged ions (red balls).
Lithium-like Fe23+
A group of scientists from the Max-Planck-Institute for Nuclear Physics in Heidelberg, the University of Hamburg and DESY has recently studied electronic transitions in Fe23+, a very abundant ion in e.g. solar flares.
Fe23+ has only three electrons left and resembles lithium apart from the much stronger electromagnetic field experienced by the electrons. As a first objective the research group investigated the transition between the (1s22s) 2S1/2 ground state and the exited (1s22p) 2P1/2 state occurring in all three-electron ions from lithium to U89+. This electronic transition is closely related to the Lamb shift in atomic hydrogen which was the key for the discovery of QED. Thus, it is no surprise that the investigated transition plays a critical role in the formulation of few-electron QED theory in the strong electromagnetic fields of highly charged ions.
The experiment was performed in a single-photon resonant excitation scheme by tuning monochromatic FEL pulses through the resonance of this transition in Fe23+ at an energy of 48.6 eV corresponding to a wavelength of 25.5 nm. The fluorescence photon yield emitted after relaxation of the excited state was registered as a function of the laser wavelength.
Beyond Theoretical Uncertainty
According to QED theory various processes contribute to the total transition energy: inter-electronic interactions, single-electron one-loop terms comprising self-energy and vacuum polarization and the corresponding screening terms.
The relative accuracy achieved in the Fe23+ experiment is already higher than the theoretical uncertainties, and the present experimental accuracy at FLASH allowed the researchers to verify the leading two-photon QED terms. Any further increase in the experimental precision will provide a systematic sensitivity improvement that may enable verification of the other processes contributing to the total transition energy.
In a few years photon energies up to 10 keV corresponding to a wavelength of 0.12 nm will become available with unprecedented flux at the LCLS and the European XFEL, and this will make it possible to investigate electronic transitions in all lithium-like ions including U89+ using the experimental method developed at FLASH.
