P. H. Mokler

Isotope shift in the dielectronic recombination of three-electron ANd57+.

Isotope shifts in dielectronic recombination spectra were studied for Li-like (A)Nd(57+) ions with A=142 and A=150. From the displacement of resonance positions energy shifts deltaE(142 150)(2s-2p(1/2))=40.2(3)(6) meV [(stat)(sys)] and deltaE(142 150)(2s-2p(3/2))=42.3(12)(20) meV of 2s-2p(j) transitions were deduced. An evaluation of these values within a full QED treatment yields a change in the mean-square charge radius of (142 150)deltar(2)=-1.36(1)(3) fm(2). The approach is conceptually new and combines the advantage of a simple atomic structure with high sensitivity to nuclear size.

Charge states and energy loss of relativistic heavy ions in matter

Abstract Relativistic heavy-ion collisions of few-electron projectiles ranging from argon up to uranium have been investigated in solid and gaseous media. Electron-loss and electron-capture cross sections, charge-state distributions, as well as energy loss and energy deposition have been measured and are compared with theoretical predictions. Especially fully-ionized heavy projectiles represent a unique possibility to test atomic-collision theories.

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.

Recombination of with free electrons at very low energies

Recombination of ions with free electrons has been investigated in a merged-beams experiment at the UNILAC of GSI in Darmstadt. Special emphasis was put on the recombination at very low energies in the electron-ion centre-of-mass frame. At we found an enormous recombination rate exceeding the theoretical expectations for radiative recombination by a factor of 365. For further investigation of this enhancement, the electron density and the magnetic field guiding the electron beam were varied. While an increase of the electron density by a factor of 10 had little influence, the measured rate coefficient increased significantly with the magnetic field strength.

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.

EXTREME-ULTRAVIOLET SPECTROSCOPY OF Fe VI-Fe XV AND ITS DIAGNOSTIC APPLICATION FOR ELECTRON BEAM ION TRAP PLASMAS

Extreme-ultraviolet spectra of intermediately ionized iron ions (Fe VI-Fe XIV) in the wavelength range of 125.0-265.0 A have been measured at the Heidelberg electron beam ion trap. Emission spectra were recorded sequentially while varying the electron energy over the range of 75-544 eV in steps of 5 eV. The observed spectra clearly show the evolution of each ionic stage as a function of the electron energy, allowing to distinguish the emission lines from neighboring ion charge species and helping to disentangle possible line blends. The collisional-radiative modeling satisfactorily reproduces the measurement. A comparison with previous astrophysical observations (Sun) reveals that some weak emissions may originate from Fe VI and Fe VII, resulting in incorrect assignment of transition lines. The calculated polarization effects due to nonthermal (monoenergetic) electrons are found to be negligible for most of the emission lines at low-energy electron impact, except for a few lines whose polarization can be over 20%. By line ratio technique, the effective electron density in the trap was estimated to be 7.1+2.4 -3.0 × 109-3.4+0.5 -0.5 × 1010 cm-3, slightly depending on the ion charge state.

Recombination of free electrons with ions

This paper reports first experimental data on radiative recombination of ions with free electrons and discusses a measurement of recombination rates of U28+ ions which appears to contradict known theories.

High-precision laser-assisted absolute determination of x-ray diffraction angles.

A novel technique for absolute wavelength determination in high-precision crystal x-ray spectroscopy recently introduced has been upgraded reaching unprecedented accuracies. The method combines visible laser beams with the Bond method, where Bragg angles (θ and -θ) are determined without any x-ray reference lines. Using flat crystals this technique makes absolute x-ray wavelength measurements feasible even at low x-ray fluxes. The upgraded spectrometer has been used in combination with first experiments on the 1s2p(1)P(1) → 1s(2)(1)S(0) w-line in He-like argon. By resolving a minute curvature of the x-ray lines the accuracy reaches there the best ever reported value of 1.5 ppm. The result is sensitive to predicted second-order QED contributions at the level of two-electron screening and two-photon radiative diagrams and will allow for the first time to benchmark predicted binding energies for He-like ions at this level of precision.

Equilibrium charge state distributions for relativistic heavy ions

Abstract Equilibrium charge-state distributions have been measured for 617.2 MeV/u Au and 758.2 MeV/u Xe projectiles traversing C, Al, and Pb targets. In all cases bare ions dominate the charge-state distributions as expected for such relativistic velocities. For both projectiles used, the Al target produced the largest fraction of bare ions. This result is in agreement with calculations and with measurements for uranium ions given in the literature. These first atomic physics measurements at the new SIS accelerator have been performed during commissioning of the fragment separator (FRS).