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 N 3+ and Ar 8+ 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 Ar 8+ .
Abstract. 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 (HCI) by soft x-ray free-electron lasers 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 HCI as a function of the wavelength. The present experiments reach already 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 1s 2 2s 2 S 1/2 -1s 2 2p 2 P 1/2 in Fe 23+ at 48.6 eV, in Cu 27+ at 55.2 eV, and 1s 2 2s 2 S 1/2 -1s 2 2p 2 P 3/2 in Fe 23+ at 65.3 eV. The latter demonstrates laser spectroscopy of multiply or highly charged ions 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 standards in this spectral region, which is still lacking.
Photoionization (PI) of Fe14+ in the range from 450 to 1100 eV was measured at the BESSY II storage ring using an electron beam ion trap achieving high target-ion area densities of 10(10) cm(-2). Photoabsorption by this ion is observed in astrophysical spectra and plasmas, but until now cross sections and resonance energies could only be provided by calculations. We reach a resolving power E/ΔE of at least 6500, outstanding in the present energy range, which enables benchmarking and improving the most advanced theories for PI of ions in high charge states.
Total electron emission yields have been measured for the first time resulting from impact of slow highly charged Arq+ (q ≤ 17), Xeq+ (q ≤ 50) and Hgq+ (q ≤ 68) ions on clean insulating LiF(001) and CaF2(111) surfaces at various impact angles. The surprisingly large yields show that even for the highest projectile charge states, a local charge-up of the surface poses no barrier for electron emission. We demonstrate that this is due to a strong sub-surface contribution in the potential electron emission process which is considerably more efficient in insulators because of the increased inelastic mean free path and the production of secondary electrons
We report the observation of trielectronic recombination with simultaneous excitation of a K-shell and an L-shell electron, hence involving three active electrons. This process was identified in the x-ray emission spectrum of recombining highly charged Kr ions. An energy resolution three times higher than any reported for this collision energy range around 10 keV resulted in the separation of the associated lines from the stronger dielectronic resonances. For Kr 30+ , intershell trielectronic recombination contributions of nearly 6% to the total resonant photorecombination rate were found.Electron-electron interaction mediates the strongest atomic processes such as dielectronic recombination ͑DR͒ and its time reversal, Auger decay, following photoexcitation of an inner-shell electron ͓1,2͔. Higher-order electron correlation plays also an essential role; however, these relevant processes are difficult to approach, both experimentally and theoretically. In the simple DR process involving only two interacting electrons, as sketched in Fig. 1 ͑left side͒, the kinetic energy of the recombined electron is transferred to a single bound electron by a radiationless excitation to an intermediate autoionizing state. The recombination is completed by its radiative stabilization. For the case of highly charged ions ͑HCIs͒, radiative transition probabilities are high, and the competition of radiative deexcitation and Auger decay of the intermediate state is biased toward the first mechanism.Beyond the well-known DR, resonant recombination processes involving higher-order correlations are relevant, too. Here, as displayed in Fig. 1, two or even three bound electrons can be simultaneously excited by the resonantly captured electron in trielectronic or even quadruelectronic recombination ͑TR and QR, respectively͒.Here, we present evidence for "intershell" TR involving excitation of a K-shell electron simultaneously with one of the L shells and provide a comparison with our theoretical predictions. TR and QR resonance energies and cross sections were predicted in the framework of the multiconfiguration Dirac-Fock ͑MCDF͒ method. Weak experimental signatures of intershell QR are also found at their calculated values. The processes we investigated are denoted as KL-LLL TR and KLL-LLLL QR, where the first set of capital letters indicates the initial shells of the bound electrons and the second one refers to the shells of the captured and excited electrons. In contrast to the already reported "intrashell" TR ͓3͔, for intershell higher-order processes the electron overlap is correspondingly smaller. Moreover, for higher atomic numbers the e-e correlation gets relatively weaker compared to the central force.Resonant mechanisms are highly efficient in either ionizing or recombining ions and hence already DR is of paramount importance for the physics of outer planetary atmospheres and interstellar clouds as well as an important radiative cooling mechanism in astrophysical and laboratory high-temperature plasmas ͓1,2,4͔. DR often represents...
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