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The dielectronic recombination rate coefficient of $\ion{O}{vi}$ ions has been measured at the heavy ion storage ring cryring. The electron energy range covered all dielectronic recombination resonances attached to $\rm 2s \rightarrow 2p$ ($\Delta n=0$) core excitations. The rate coefficient in a plasma has been derived. It is compared to theoretical data. In addition the influence of external electric fields on the dielectronic recombination has been investigated. When increasing the electric field strength to 340 V/cm the experimental recombination rate coefficient is found to increase by up to a factor of 2.

One ionization mechanism of barium ions is the resonant excitation of a 3d inner-shell electron with the subsequent decay of the resulting highly excited intermediate state by multiple autoionization processes. We have experimentally studied this process by observing associated resonance structures – in the energy range 700–800 eV – in the multiple (up to sevenfold) ionization cross-sections of Baq+ ions with 1⩽q⩽13. The 3d-resonance strength is determined quantitatively as a function of initial and final ion charge state. The results suggest a statistical description of multiple ionization via the excitation of inner-shell electrons.

We report on a design study of a high-current electron gun producing a ribbon-shaped electron beam. The gun is designed for the energy range of 10–3500 eV. It has a perveance of 3.9×10 −6 AV −3/2 . The gun consists of nine separate electrodes. In the interaction region a parallel electron beam with a cross section of about 2 mm × 60 mm and a current density as high as 1.0 A/cm 2 is provided. The electron gun offers features especially desirable in electron–ion collision experiments. It can operate in different modes allowing for the variation of the electron current with or without a potential trap in the interaction region at any desired energy within the design range.

The principle of microscopic time-reversal of physical processes, detailed balance, is widely used and depended upon in the theoretical community as a connection between the inverse processes of photorecombination (PR) and photoionization (PI). This paper reports on a novel use of detailed balance and the comparison of experimental results obtained by two very distinct techniques to determine both the metastable fraction of an ion beam and partial Sc3+ PR cross sections and partial Sc2+ PI cross sections for the ground state and for two metastable states. The Sc2+ to Sc3+ system presents a unique opportunity to obtain comprehensive state-selective information by combining results from PR and PI experiments.

Field-enhanced dielectronic recombination (DR) cross-sections differential in the main quantum number n have been measured for N4+ ions. The n-differential data were obtained at the heavy ion storage ring CRYRING by increasing the ion energy in two steps from 3 to 5 MeV/u and from 5 to 7.5 MeV/u. The accompanying increase of the magnetic field in the storage – ring dipole magnets leads to a decrease of the cutoff quantum number nc associated with the field ionization of intermediate N3+(1s22p nl) states. By comparing measurements at two different ion energies the enhancement of DR due to electromagnetic fields is derived for the narrow bandwidths of Rydberg states n≈17–21 and 19–24. The applied electric fields ranged from 0 to 300 V/cm. The observed enhancement was up to a factor of 1.9 for the bandwidth n≈17–21 and up to 2.3 for the bandwidth n≈19–24, respectively.

We have measured resonance strengths and energies for dielectronic recombination (DR) of Fe XIX forming Fe XVIII via N = 2 → N' = 2 and N = 2 → N' = 3 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We have also calculated these resonance strengths and energies using two independent, state-of-the-art techniques: the perturbative multiconfiguration Breit-Pauli (MCBP) and multiconfiguration Dirac-Fock (MCDF) methods. Overall, reasonable agreement is found between our experimental results and theoretical calculations. The most notable discrepancies are for the 3l3l' resonances. The calculated MCBP and MCDF resonance strengths for the n = 3 complex lie, respectively, ≈47% and ≈31% above the measured values. These discrepancies are larger than the estimated 20% total experimental uncertainty in our measurements. We have used our measured 2 → 2 and 2 → 3 results to produce a Maxwellian-averaged rate coefficient for DR of Fe XIX. Our experimentally derived rate coefficient is estimated to be good to better than ≈20% for kBTe ≥ 1 eV. Fe XIX is predicted to form in photoionized and collisionally ionized cosmic plasmas at kBTe 1 eV. Hence, our rate coefficient is suitable for use in ionization balance calculations of these plasmas. Previously published theoretical DR rate coefficients are in poor agreement with our experimental results. None of these published calculations reliably reproduce the magnitude or temperature dependence of the experimentally derived rate coefficient. Our MCBP and MCDF results agree with our experimental rate coefficient to within ≈20%.

We have measured the resonance strengths and energies for dielectronic recombination (DR) of Fe XX forming Fe XIX via N=2 --> N'=2 (Delta_N=0) core excitations. We have also calculated the DR resonance strengths and energies using AUTOSTRUCTURE, HULLAC, MCDF, and R-matrix methods, four different state-of-the-art theoretical techniques. On average the theoretical resonance strengths agree to within ~30% which is significantly larger than the estimated relative experimental uncertainty of ~1 eV, which includes the predicted formation temperatures for Fe XX in an optically thin, low-density photoionized plasma with cosmic abundances, our experimental and theoretical results are in good agreement. We have also used our R-matrix results, topped off using AUTOSTRUCTURE for RR into J>=25 levels, to calculate the rate coefficient for RR of Fe XX. Our RR results are in good agreement with previously published calculations.

XXX International Conference on Photonic, Electronic, and Atomic Collisions, Cairns, Australia, 25 Jul 2017 - 1 Aug 2017 ; Journal of physics / Conference Series 875, 022004(2017). doi:10.1088/1742-6596/875/3/022004, Absolute cross sections for double and triple photodetachment of O- ions have been measured at photon energies around the threshold for K-shell ionization using the photon-ion merged-beams technique at a synchrotron light-source. In addition, corresponding large-scale ab-initio calculations were carried out at a level of complexity that has never been invoked before. From the comparison between experiment and theory it becomes apparent that the inclusion of multi-electron processes such as double shake-up in the theoretical calculations is crucial for the explanation of the experimental findings., Published by IOP Publ., Bristol

A high-power electron gun delivering a ribbon-shaped electron beam with energies of 10-3500 eV and high currents at all energies is currently being commissioned at the electron-ion crossed-beams setup in Giessen. As compared to the formerly used electron gun [1], the new one will enable us to perform measurements of cross-sections for electron-impact ionization of ions in a wider energy range and, consequently, for ion charge states that were not accessible before.

We present experimentally measured and theoretically calculated rate coefficients for the electron-ion recombination of W18+([Kr]4d104f10) forming W17+. At low electron-ion collision energies, the merged-beam rate coefficient is dominated by strong, mutually overlapping recombination resonances. In the temperature range where the fractional abundance of W18+ is expected to peak in a fusion plasma, the experimentally derived Maxwellian recombination rate coefficient is 5 to 10 times larger than that which is currently recommended for plasma modeling. The complexity of the atomic structure of the open-4f system under study makes the theoretical calculations extremely demanding. Nevertheless, the results of the present Breit-Wigner partitioned dielectronic recombination calculations agree reasonably well with the experimental findings. This also gives confidence in the ability of the theory to generate sufficiently accurate atomic data for the plasma modeling of other complex ions.

We report on experimental and theoretical results for the photoionization of Ag-like xenon ions, Xe$^{7+}$, in the photon energy range 95 to 145~eV. The measurements were carried out at the Advanced Light Source at an energy resolution of $\Delta$E = 65 meV with additional measurements made at $\Delta$E = 28 meV and 39 meV. Small resonance features below the ground-state ionization threshold, at about 106 eV, are due to the presence of metastable Xe$^{7+} (4d^{10} 4f~^2{\rm F}^{\circ}_{5/2,7/2})$ ions in the ion beam. On the basis of the accompanying theoretical calculations using the Dirac Atomic R-matrix Codes (DARC), an admixture of only a few percent of metastable ions in the parent ion beam is inferred, with almost 100\% of the parent ions in the $(4d^{10}5s ~^2{\rm S_{1/2}})$ ground level. The cross-section is dominated by a very strong resonance associated with $4d \rightarrow 5f$ excitation and subsequent autoionization. This prominent feature in the measured spectrum is the $4d^95s5f ~^2{\rm P}^{\circ}$ resonance located at (122.139 $\pm$ 0.01)~eV. An absolute peak cross-section of 1.2 Gigabarns was measured at 38 meV energy resolution. The experimental natural width $\Gamma$ = 76 $\pm$ 3 meV of this resonance compares well with the theoretical estimate of 88 meV obtained from the DARC calculation with 249 target states. Given the complexity of the system, overall satisfactory agreement between theory and experiment is obtained for the photon energy region investigated., Comment: 19 pages, 6 figures, accepted to appear in J. Phys. B: At. Mol. & Opt. Phys

The radiative electron capture of a target electron into the projectile continuum has been studied for the collision system ${\text{U}}^{88+}+{\text{N}}_{2}\ensuremath{\rightarrow}{\text{U}}^{88+}+{[{\text{N}}_{2}^{+}]}^{*}+{e}^{\ensuremath{-}}+\ensuremath{\gamma}$ at 90 MeV/$\mathrm{u}$. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted under an angle of ${0}^{\ensuremath{\circ}}$ with respect to the projectile beam and with a velocity close to the projectile velocity has been measured in coincidence with the emitted photons under various observation angles. The experimental results provide a stringent test for the corresponding process in inverse kinematics, namely, the theory of electron-nucleus bremsstrahlung at the high-energy endpoint. For comparison this process is calculated using fully relativistic Dirac wave functions and using semirelativistic Sommerfeld-Maue wave functions.

Single, double, and triple ionization of C(1+) ions by single photons is investigated in the energy range of 286-326 eV, i.e., in the range from the lowest-energy K-vacancy resonances to well beyond the K-shell ionization threshold. Clear signatures of C(1+)(1s2s(2)2p(2) (2)D,(2)P) resonances are found in the triple-ionization channel. The only possible mechanism producing C(4+)(1s(2)) via these resonances is direct triple-Auger decay, i.e., a four-electron process with simultaneous emission of three electrons.

The role of hollow states intermediately produced in electron-impact ionization of metastable He-like $N{}^{5+}(1s2s{\phantom{\rule{0.16em}{0ex}}}^{3}{S}_{1})$ ions has been investigated in detail. A crossed-beam setup and suitable experimental techniques were employed for the measurement of accurate absolute cross sections and precise energy-scan data. Fine structures arising from $K$-shell excitations and associated resonances have been observed for this two-electron ion with less than $\ifmmode\pm\else\textpm\fi{}$0.5 eV uncertainty on the energy scale. Fine details, such as interference of the reaction pathways of direct ionization and excitation with capture of the incident electron followed by double-Auger decay, could be revealed. Ab initio calculations based on the convergent close coupling (CCC) approach are in good agreement with the experiment.

We have measured dielectronic recombination (DR) for Fe12 + forming Fe11 + using the heavy ion storage ring TSR located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Using our results, we have calculated a plasma rate coefficient from these data that can be used for modeling astrophysical and laboratory plasmas. For the low temperatures characteristic of photoionized plasmas, the experimentally derived rate coefficient is orders of magnitude larger than the previously recommended atomic data. The existing atomic data were also about 40% smaller than our measurements at temperatures relevant for collisionally ionized plasmas. Recent state-of-the-art theory has difficulty reproducing the detailed energy dependence of the DR spectrum. However, for the Maxwellian plasma rate coefficient, recent theoretical results agree with our measurements to within about 30% for both photoionized and collisionally ionized plasmas.

Absolute cross sections for the K-shell photoionization of Be-like boron ions were measured with the ion-photon merged-beams technique at the Advanced Light Source synchrotron radiation facility. High-resolution spectroscopy with E/$\Delta$E up to 8800 ($\Delta$E $\sim$ 22 meV) covered the energy ranges 193.7 -- 194.7 eV and 209 -- 215 eV. Lifetimes of the strongest resonances are determined with relative uncertainties down to approximately 4% for the broadest resonance. The measured resonance strengths are consistent with 60 % $\rm 1s^22s^2$ $^1S$ ground-state and 40 % $\rm 1s^22s2p$ $^3P^o$ metastable-state ions in the primary ion beam and confirmed by comparison with independent absolute photo-recombination heavy-ion storage-ring measurements with B$^{2+}$ ions using the principle of detailed balance. Experimental determination of the line width for the $\rm 1s2s^22p$ $^1$P$^{\rm o}$ resonance gives a value of 47 $\pm$ 2 meV and compares favourably to a theoretical estimate of 47 meV from the R-matrix with pseudo-states (RMPS) method. The measured line widths of the $\rm 1s2s2p^2$ $^3$P, $^3$D resonances are 10.0 $\pm$ 2 meV and 32 $\pm$ 3meV, respectively, compared to RMPS theoretical estimates of 9 meV and 34 meV., Comment: 23 pages, 6 figures, 2 tables, accepted for publication in J. Phys. B

The photon-ion merged-beams technique has been employed at the new Photon-Ion spectrometer at PETRA III (PIPE) for measuring multiple photoionization of Xe$^{q+}$ (q=1-5) ions. Total ionization cross sections have been obtained on an absolute scale for the dominant ionization reactions of the type h\nu + Xe$^{q+}$ $\to$ Xe$^{r+}$ + (q-r) e$^-$ with product charge states q+2 $\le$ r $\le$ q+5. Prominent ionization features are observed in the photon-energy range 650-750 eV, which are associated with excitation or ionization of an inner-shell 3d electron. Single-configuration Dirac-Fock calculations agree quantitatively with the experimental cross sections for non-resonant photoabsorption, but fail to reproduce all details of the measured ionization resonance structures., Comment: 20 pages, 12 figures, submitted for publication to J. Phys. B

Single ionization of noble-gas atoms by linearly polarized synchrotron radiation has been studied by employing angle- and energy-resolved photoelectron spectroscopy. The measurements were carried out in the plane defined by the momentum and polarization vectors of the photon. Parameters describing the left-right asymmetry (LRA; relative to the photon propagation direction) of the photoelectron angular distribution were determined experimentally for the $s$ shells of He, Ne, Ar, Kr, and Xe atoms and $\mathrm{H}{}_{2}$ molecules and for the $p$ shells of Ne, Ar, Kr, and Xe atoms. The values of the left-right asymmetry differ significantly from zero for both subshells. The photon and photoelectron energy dependence of the LRA parameters is presented also. Possible experimental and instrumental sources that could generate asymmetry are discussed and excluded as well.

Journal of physics / Conference Series 488(14), 142006 (2014). doi:10.1088/1742-6596/488/14/142006, A photon-ion merged beam endstation has been set up at the variable polarization XUV-beamline P04 of PETRA III in Hamburg. In a commissioning experiment first results could be obtained for multiple photoionization of Xe$^{q+}$ ions (q = 1,2,..,5) at photon energies around the 3d ionization threshold., Published by IOP Publ., Bristol

Recombination of fully stripped U92+ ions with electrons has been investigated at the Experimental Storage Ring (ESR) in Darmstadt. Absolute recombination rate coefficients have been measured for relative energies from 0 to 33 eV. For energies greater than 20 meV the experimental result is well described by the theory for radiative recombination (RR). Below 20 meV the experimental rate increasingly exceeds the RR calculation as observed previously in the recombination of light bare ions as well as of Bi83+. This low-energy rate enhancement is shown to scale as Z2.6 for bare ions, where Z is the atomic number of the ion. The U92+ recombination rate enhancement is insensitive to changes of the electron density. Variation of the magnetic guiding field strength from 80 mT to 120 mT resulted in oscillations of the recombination rate at 0 eV. The oscillations are partly attributed to changes of the transverse electron temperature accompanying the change of the magnetic guiding field strength; partly they may be caused by uncompensated small changes of the interaction angle between the two beams.

Radiative recombination (inverse photoionization) is believed to be well understood since the beginning of quantum mechanics. Still, modern experiments consistently reveal excess recombination rates at very low electron-ion center-of-mass energies. In a detailed study on recombination of F and C ions with magnetically guided electrons we explored the yet unexplained rate enhancement, its dependence on the magnetic field B, the electron density n(e), and the beam temperatures T-perpendicular to and T-parallel to. The excess scales as T-perpendicular to(-1/2) and, surprisingly, as T-parallel to(-1/2), increases strongly with B, and is insensitive to n(e). This puts strong constraints on explanations of the enhancement.

In the search for interference between radiative and dielectronic recombination (RR and DR), absolute recombination rate coefficients for Ar-like ions have been measured at the Heidelberg Test Storage Ring in the centre-of-mass energy range 0-80 eV. The dominant recombination channels beside RR is DR via the formation of (33d ) and (33d ) intermediate doubly excited states. The DR rate coefficient in plasmas is inferred from this measurement. It is about a factor of 3 lower than the previously available result based on a semi-empirical calculation. Asymmetric lineshapes due to quantum mechanical interference as recently predicted theoretically for isoelectronic ions have not been observed. The broad (3) DR resonance expected on the basis of multi-configuration Hartree-Fock calculations at 3.0 eV with a width of 1.3 eV appears to be shifted towards zero centre-of-mass energy where an unexplained recombination rate enhancement of a factor of 2 beyond the sum of RR and DR rates is observed.

New recombination experiments with merged cold beams of electrons and atomic ions have been carried out at the storage ring facilities TSR in Heidelberg, ESR in Darmstadt, and CRYRING in Stockholm. A brief overview is given on the recent activities in which the Giessen group was engaged. Topics of this research were dielectronic recombination (DR) of astrophysically relevant ions, recombination of highly charged ions with respect to cooling losses in storage rings, field effects on DR, search for interference effects in photorecombination of ions, correlation effects in DR of low-Z ions, spectroscopy of high-Z ions by DR, and lifetimes of metastable states deduced from DR experiments.

Recombination of Ar-like Sc3+ ions has been studied experimentally at the TSR for center of mass energies ranging from approximately 12–18 eV. In this energy range rather broad resonances from short lived Sc2+(3s23p53d2 2F) intermediate states are expected. For these resonances recent theoretical work predicts strongly asymmetric lineshapes due to quantum mechanical interferences between radiative and dielectronic recombination channels. The study of this heavy, low-charged ion suffered from a high background due to electron capture and the outcome of our experiment does not yet allow for a conclusive test of the theoretical predictions.

The origin of the widely observed enhancement of rates for electron-ion recombination at very low energies is still unknown. We investigated the recombination of Au25+ with free electrons in a merged-beams experiment at the UNILAC accelerator of the GSI in Darmstadt. At Erel= 0 eV we found an enormous enhancement factor of 365 compared to the theory of radiative recombination. An increase of the electron density by a factor of 10 had not much influence on the measured rate coefficient.

Dielectronic recombination (DR) and radiative recombination (RR) of lithium-like gold in the energy range of 0 to 225 eV have been studied at the Experimental Storage Ring (ESR) of the GSI in Darmstadt. Main objective of the measurements is the precise determination of the 2s1/2−2p1/2 energy splitting as an additional QED test. This novel method, developed at the ESR [1], is based on the extrapolation of a multitude of measured resonances Au75+ (1s22p1/2nlj) up to the series limit (n = ∞). Furthermore experimental data for the Au75+ (1s22p3/26lj) resonance manifold are presented.

Double photoionization accompanied by loss of n C atoms (n=0, 2, 4, 6) was investigated by merging beams of Xe@C60+ ions and synchrotron radiation and measuring the yields of product ions. The giant 4d dipole resonance of the caged Xe atom has a prominent signature in the cross section for these product channels, which together account for 6.2 ± 1.4 of the total Xe 4d oscillator strength of 10. Compared to that for a free Xe atom, the oscillator strength is redistributed in photon energy due to multipath interference of outgoing Xe 4d photoelectron waves that may be transmitted or reflected by the spherical C60+ molecular cage, yielding so-called confinement resonances. The data are compared with an earlier measurement and with theoretical predictions for this single-molecule photoelectron interferometer system. Relativistic R-matrix calculations for the Xe atom in a spherical potential shell representing the fullerene cage show the sensitivity of the interference pattern to the molecular geometry. © 2013 American Physical Society.

At the FAIR facility for antiproton and ion research, the new ESR + CRYRING combination of storage rings CRYRING@ESR opens up a wealth of opportunities for in-ring atomic physics experiments on few-body quantum dynamics. The low-energy storage ring CRYRING will serve in its new location at FAIR/ESR for experiments with decelerated antiprotons and highly charged ions. We will discuss selected new experiments in the field of quantum dynamics of high-Z ions, for example for adiabatic superheavy quasi-molecules transiently formed with bare and H-like projectiles. Such experiments will be for the first time possible at the future CRYRING at ESR.

We have obtained experimental photo-double- and photo-triple-detachment cross sections for the fullerene negative ion using Advanced Light Source photons of 17-90 eV. The cross sections are 2 and 2.5 times larger than those for C60 and appear to be compressed and shifted in photon energy as compared to C60. Our analysis reveals that the additional electron in C60- primarily produces screening which is responsible for the modification of the spectrum. Both screening effects, the shift and the compression, can be quantitatively accounted for by a linear transformation of the energy axis. Applying the transformation allows us to map the neutral and negative ion cross sections onto each other, pointing out the close relationship of correlated few-electron dynamics in neutral and negatively charged extended systems. In contrast, dynamics of neutral and negatively charged atoms or small molecules are typically not closely related.

In this work we report on ab initio theoretical results for the magnetic field induced 2s2p ^3P_0 - 2s^2 ^1S_0 E1 transition for ions in the beryllium isoelectronic sequence between Z=5 and 92. It has been proposed that the rate of the E1M1 two-photon transition 2s2p ^3P_0 - 2s^2 ^1S_0 can be extracted from the lifetime of the ^3P_0 state in Be-like ions with zero nuclear spin by employing resonant recombination in a storage-ring. This experimental approach involves a perturbing external magnetic field. The effect of this field needs to be evaluated in order to properly extract the two-photon rate from the measured decay curves. The magnetic field induced transition rates are carefully evaluated and it is shown that, with a typical storage-ring field strength, it is dominant or of the same order as the E1M1 rate for low- and mid-Z ions. Results for several field strengths and ions are presented and we also give a simple Z-dependent formula for the rate. We estimate the uncertainties of our model to be within 5% for low- and mid-Z ions, and slightly larger for more highly charged ions. Furthermore we evaluate the importance of including both perturber states, ^3P_1 and ^1P_1, and it is shown that excluding the influence of the ^1P_1 perturber overestimates the rate by up to 26% for the mid-Z ions., Comment: 21 pages, 5 figures

We report measurements of electron impact ionization (EII) for Fe^13+, Fe^16+, and Fe^17+ over collision energies from below threshold to above 3000 eV. The ions were recirculated using an ion storage ring. Data were collected after a sufficiently long time that essentially all the ions had relaxed radiatively to their ground state before data were collected. For single ionization of $\fethirteen$ we find that previous single pass experiments are more than 40% larger than our results. Compared to our work, the theoretical cross section recommended by Arnaud & Raymond (1992) is more than 30% larger, while that of Dere (2007) is about 20% greater. Much of the discrepancy with Dere (2007) is due to the theory overestimating the contribution of excitation-autoionization via n=2 excitations. Double ionization of Fe^13+ is dominated by direct ionization of an inner shell electron accompanied by autoionization of a second electron. Our results for single ionization of Fe^16+ and Fe^17+ agree with theoretical calculations to within the experimental uncertainties., Accepted for the Astrophysical Journal

Absolute cross sections for C3+(6-->5) emission at 465.7 nm after collisions of C4+ ions with ground state Na(3s) and laser excited aligned Na(Sp) atoms are measured over the collision energy range of 3-7 keV amu(-1). For Na(3s) polarizations are observed by measuring the linear polarization of the emitted light. In these collisions the polarization is found to be 0.33 and almost independent of the collision energy. Cross sections for collisions with the Na(3p) target are up to five times larger than for collisions with Na(3s). Slightly larger cross sections are measured when the laser excited Na(3p) target is Pi-aligned as compared to a Sigma-alignment. Classical trajectory Monte Carlo (CTMC) calculations agree excellently with the experimental findings and yield predictions of strong positive polarizations for electron capture also from aligned Na(3p), indicating preferential capture to m(l) = 0 states.

We investigated l and m-distributions of He II (n = 4) states formed during 2-13.3 keV amu(-1) He2+ + Na(3s) collisions by measuring profiles and the linear polarization of the He II (4 --> 3) emission. We found that at these energies capture into He+(4f) dominates over capture into other He+(4l) states. Over the indicated energy range the polarization of the He-4 II (4 --> 3) photon emission increases from 0.2 to 0.3 indicating a strong alignment of the captured projectile charge cloud along the internuclear axis. Due to the non-zero nuclear spin the polarization is up to 10% lower when using He-3(2+) projectiles instead of He-4(2+). Classical trajectory Monte Carlo calculations agree quantitatively with these experimental findings and predict a decrease of the polarization towards zero at higher energies due to enhanced capture into He+(4s).A positive polarization is also found for the Na I (3p --> 3s) target de-excitation. However, due to strong depolarization effects caused by the nuclear spin of the Na-23 nucleus it stays below 0.14.

The authors have measured scattered projectile angular and charge state distributions for 3.75 keV/amu O{sup q+} (3 {le} q {le} 8) and 1.2 keV/amu Ar{sup 1+} (3 {le} q {le} 14) ions grazingly incident along the [110] and [100] directions of a Au(110) single crystal target. Scattered projectile angular distribution characteristic of surface channeling are observed. For both incident species, the dominant scattered charge fraction is neutral, which varies only by a few percent as a function of incident charge state. Significant O{sup {minus}} formation is observed, which manifests a distinct velocity threshold. For incident Ar projectiles with open L-shells, the positive scattered charge fractions, while always less than about 10%, increase linearly with increasing number of initial L-shell vacancies.

Recent Oak Ridge work is summarized on projectile energy gain by image charge acceleration, scattered ion charge distributions, and K-Auger electron emission during low energy grazing interactions of highly charged Pb, I, O and Ar ions with a Au(110) surface.

This paper presents a detailed description of {ital KLL} Auger spectra arising from collisions of hydrogenic N{sup 6+} ions on clean Si(100) and Al(110) surfaces at energies ranging from 78 eV up to 60 keV and at incident angles from 2.5{degree} up to 45{degree}. Atomic structure calculations of {ital KLL} Auger energies and decay rates for hollow atoms, together with simulations of the projectile trajectories, support a model in which the de-excitation of the ions proceeds essentially via two mechanisms. Before the ion enters the ``close collision range,`` a few of its {ital L}-shell vacancies are filled by Auger cascades involving outer shell electrons. But as seen as the projectile suffers close collisions, {ital L}-shell vacancies are rapidly filled by direct capture of target core electrons. The time scale of the latter mechanism is determined by the collision frequency.

Recent collision experiments are reviewed in which one-electron transfer between laser excited target atoms and (highly charged) keV-ions has been studied. Especially results showing a dependence of the charge exchange on the initial target orbital alignment are discussed. The question to what extent simple intuitive physical pictures such as velocity matching can explain the experimental findings is addressed.

We report on measurements of scattered projectile angular and charge state distributions for 3.75 keV/amu O{sup {ital q}+} (3{le}{ital q}{le}8) ions surface channeled along the {l_angle}1{bar 1}0{r_angle} direction of a Au(110) single crystal target. Projectile trajectory calculations indicate that the projectiles spend less than 30 fs within 2 A of the topmost Au surface layer. Nevertheless, almost complete charge equilibration is observed in the scattered projectile charge state distributions. It is concluded that, {ital already} {ital at} {ital the} {ital target} {ital interface}, strong screening effects are present which make possible direct capture to the projectile {ital L} and {ital M} shells in relatively large impact parameter collisions.

We have measured electron impact ionization (EII) for Fe8+ forming Fe9+ from below the ionization threshold to 1200 eV. These measurements were carried out at the TSR heavy ion storage ring. The objective of using a storage ring is to store the ion beam initially so that metastable levels decay, thereby allowing for measurements on a well-defined ground-level ion beam. In this case, however, some metastable levels were too long lived to be removed. We discuss several methods for quantifying the metastable fraction, which we estimate to be ~30%–40%. Although metastables remain problematic, the present storage ring work improves upon other experimental geometries by limiting the metastable contamination to only a few long-lived excited levels. We discuss some future prospects for obtaining improved measurements of Fe8+ and other ions with long-lived metastable levels.

We report measured rate coefficients for electron-ion recombination for Si10+ forming Si9+ and for Si9+ forming Si8+, respectively. The measurements were performed using the electron-ion merged-beams technique at a heavy-ion storage ring. Electron-ion collision energies ranged from 0 to 50 eV for Si9+ and from 0 to 2000 eV for Si10+, thus, extending previous measurements for Si10+ [Orban et al. 2010, Astrophys. J. 721, 1603] to much higher energies. Experimentally derived rate coefficients for the recombination of Si9+ and Si10+ ions in a plasma are presented along with simple parameterizations. These rate coefficients are useful for the modeling of the charge balance of silicon in photoionized plasmas (Si9+ and Si10+) and in collisionally ionized plasmas (Si10+ only). In the corresponding temperature ranges, the experimentally derived rate coefficients agree with the latest corresponding theoretical results within the experimental uncertainties., Comment: 17 pages, 7 figures, 3 tables, 66 references, submitted to the J. Phys. B special issue on atomic and molecular data for astrophysicists

Experimental and theoretical results are reported for single-photon single ionization of and W3+ tungsten ions. Experiments were performed at the photon–ion merged-beam setup of the Advanced Light Source in Berkeley. Absolute cross sections and detailed energy scans were measured over an energy range 20–90 eV at a bandwidth of 100 meV. Broad peak features with widths typically around 5 eV have been observed with almost no narrow resonances present in the investigated energy range. Theoretical results were obtained from a Dirac–Coulomb R-matrix approach. The calculations were carried out for the lowest-energy terms of the investigated tungsten ions with levels for and for . Assuming a statistically weighted distribution of ions in the initial ground-term levels there is good agreement of theory and experiment for ions. However, for ions at higher energies there is a factor of approximately two difference between experimental and theoretical cross sections.

The absolute electron-impact ionization cross section of Sc + ions was measured by employing the crossed beams technique. We find pronounced contributions (up to ∼30%) of indirect processes to the total ionization cross section. This finding is related to the existence of exceptionally strong 3p→3d excitation channels in Sc + (3p 6 3d4s).

Absolute rate coefficients for the dielectronic recombination (DR) of Na 8+ ions producing doubly excited states in beryllium-like Na 7+ have been measured at the Heidelberg heavy-ion test storage ring (TSR). The investigated electron energy range covers all DR resonances related to 2s→2p 1/2 and 2s→2p 3/2 Δ n =0 core excitations. Resonances related to Δ n =1 and Δ n =2 transitions of the valence electron are found at relative energies between 50 and 250 eV. At very low energies the resonance group associated with Na 7+ (1s 2 2p j 7l j ′ ) intermediate states has been investigated with an energy resolution and precision that will challenge state-of-the-art theory.

Recombination between Be-like ions and free electrons was measured using the merged-beams technique at the TSR storage ring. Trielectronic recombination via triply excited intermediate states was found to contribute significantly to the recombination spectrum. The strength of this process relies strongly on the absence of continua of singly-excited core states. Calculations corroborate the experimental findings.

Fluorescence of iron ions induced by an X-ray laser allows the relative oscillator strength for Fe xvii emission to be determined; it is found to differ by 3.6σ from the best quantum mechanical calculations, suggesting that the poor agreement between prediction and observations of the brightest Fe xvii line is rooted in the quality of the underlying atomic wavefunctions used in the models. The interpretation of some of the spectral data from the Chandra and XMM-Newton orbiting X-ray missions has been complicated by discrepancies between theory and observation involving the emission lines from the highly charged Fe16+ ion, also known as Fe XVII. Specifically, the intensity of the strongest Fe XVII line, one of the brightest X-ray emissions from galaxies and stars, is generally weaker than predicted. Sven Bernitt et al. report the results of laboratory experiments in which a target of iron ions was fluoresced with femtosecond X-ray pulses from a free-electron laser. They find a relative oscillator strength that differs by 3.6σ from the best quantum mechanical calculations, suggesting that the poor agreement is rooted in the calculations of the underlying atomic dynamics and that the current astrophysical models are not at fault. Highly charged iron (Fe16+, here referred to as Fe xvii) produces some of the brightest X-ray emission lines from hot astrophysical objects1, including galaxy clusters and stellar coronae, and it dominates the emission of the Sun at wavelengths near 15 angstroms. 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 predicted2,3. This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions1, 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 laser4; 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.

We have measured electron-ion recombination for Fe XII forming Fe XI using a merged beams configuration at the heavy-ion storage ring TSR located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. The measured merged beams recombination rate coefficient (MBRRC) for collision energies from 0 to 1500 eV is presented. This work uses a new method for determining the absolute MBRRC based on a comparison of the ion beam decay rate with and without the electron beam on. For energies below 75 eV, the spectrum is dominated by dielectronic recombination (DR) resonances associated with 3s-3p and 3p-3d core excitations. At higher energies we observe contributions from 3-N' and 2-N' core excitations DR. We compare our experimental results to state-of-the-art multi-configuration Breit-Pauli (MCBP) calculations and find significant differences, both in resonance energies and strengths. We have extracted the DR contributions from the measured MBRRC data and transformed them into a plasma recombination rate coefficient (PRRC) for temperatures in the range of 10^3 to 10^7 K. We show that the previously recommended DR data for Fe XII significantly underestimate the PRRC at temperatures relevant for both photoionized plasmas (PPs) and collisionaly ionized plasmas (CPs). This is to be contrasted with our MCBP PRRC results which agree with the experiment to within 30% at PP temperatures and even better at CP temperatures. We find this agreement despite the disagreement shown by the detailed comparison between our MCBP and experimental MBRRC results. Lastly, we present a simple parameterized form of the experimentally derived PRRC for easy use in astrophysical modelling codes., April 24, 2012: accepted for publication in The Astrophysical Journal