Showing papers by "Alfred Müller published in 2008"
TL;DR: An overview of the processes involved in electron-ion collisions, their cross sections, the experimental approaches and typical results can be found in this paper, where the present status of the field is discussed and characterized.
Abstract: Electron–ion collisions are among the most important atomic processes in ionized gases. They comprise elastic scattering, excitation, ionization, and recombination. Theoretical approaches are numerous but calculating reliable results for applications is a task that has not generally been solved, although theory has progressed considerably in the last decade and can now provide reference data for electron collisions with few-electron systems. Experimental data on electron–ion collisions can be inferred from plasma observations, from ion trap measurements, from careful preparation and analysis of ion–atom collisions, and from colliding-beams experiments. The spectrum of equipment employed in the measurements ranges from table top size to large accelerator and storage-ring facilities. This article provides an overview of the processes involved in electron–ion collisions, their cross sections, the experimental approaches and typical results. The present status of the field is discussed and characterized by numerous examples illuminating the accomplishments as well as the limitations of experimental access to accurate detailed cross sections and rate coefficients, as they are required for understanding and modelling laboratory or astrophysical plasmas.
151 citations
TL;DR: Term energies for dielectronic-recombination Rydberg resonances below0.07 eV are determined for Sc18+ with absolute accuracies below 0.0002 eV by electron collision spectroscopy in an ion storage ring using the twin-electron-beam technique and a cryogenic photocathode.
Abstract: Screened Radiative Corrections from Hyperfine-Split Dielectronic Resonances in Lithiumlike Scandium
142 citations
TL;DR: In this article, mass-selected beams of atomic ions were employed to study photoionization of free and encaged cerium atoms, and the inner shell contributions to single and double ionization of the endohedral fullerene cage surrounding the Ce atom were extracted from the data and compared with expectations based on theory and the experiments with atomic Ce ions.
Abstract: Mass-selected beams of atomic ${\mathrm{Ce}}^{q+}$ ions ($q=2$, 3, 4), of ${\mathrm{C}}_{82}^{+}$ and of endohedral $\mathrm{Ce}@{\mathrm{C}}_{82}^{+}$ ions were employed to study photoionization of free and encaged cerium atoms. The Ce $4d$ inner-shell contributions to single and double ionization of the endohedral $\mathrm{Ce}@{\mathrm{C}}_{82}^{+}$ fullerene have been extracted from the data and compared with expectations based on theory and the experiments with atomic Ce ions. Dramatic reduction and redistribution of the ionization contributions to $4d$ photoabsorption is observed. More than half of the Ce $4d$ oscillator strength appears to be diverted to the additional decay channels opened by the fullerene cage surrounding the Ce atom.
66 citations
TL;DR: It turns out that association, positive orthant dependence and positive supermodular dependence of Lévy processes can be characterized in terms of the LÉvy measure as well as in termsof the Lé Ivy copula.
Abstract: In this paper we investigate dependence properties and comparison results for multidimensional Levy processes. In particular we address the questions, whether or not dependence properties and orderings of the copulas of the distributions of a Levy process can be characterized by corresponding properties of the Levy copula, a concept which has been introduced recently in Cont and Tankov (Financial modelling with jump processes. Chapman & Hall/CRC, Boca Raton, 2004) and Kallsen and Tankov (J Multivariate Anal 97:1551–1572, 2006). It turns out that association, positive orthant dependence and positive supermodular dependence of Levy processes can be characterized in terms of the Levy measure as well as in terms of the Levy copula. As far as comparisons of Levy processes are concerned we consider the supermodular and the concordance order and characterize them by orders of the Levy measures and by orders of the Levy copulas, respectively. An example is given that the Levy copula does not determine dependence concepts like multivariate total positivity of order 2 or conditionally increasing in sequence. Besides these general results we specialize our findings for subfamilies of Levy processes. The last section contains some applications in finance and insurance like comparison statements for ruin times, ruin probabilities and option prices which extends the current literature.
41 citations
TL;DR: In this article, the photorecombination rate coefficients of potassium-like Fe viii and argon-like F viii were measured by employing the merged electron-ion beams method at the Heidelberg heavy-ion storagering TSR.
Abstract: The photorecombination rate coefficients of potassium-like Fe viii ions forming calcium-like Fe vii and of argon-like Fe ix forming potassium-like Fe viii were measured by employing the merged electron-ion beams method at the Heidelberg heavy-ion storagering TSR. New theoretical calculations with the AUTOSTRUCTURE code were carried out for dielectronic recombination (DR) and trielectronic recombination (TR) for both ions. We compare these experimental and theoretical results and also compare with previously recommended rate coefficients. The DR and TR resonances were experimentally investigated in the electron-ion collision energy ranges 0–120 eV and 0–151 eV for Fe viii and Fe ix. Experimentally derived Fe viii and Fe ix DR + TR plasma rate coefficients are provided in the temperature range kBT = 0.2 to 1000 eV. Their uncertainties amount to <±26% and <±35% at a 90% confidence level for Fe viii and Fe ix, respectively.
27 citations
01 Jan 2008
6 citations
29 May 2008
TL;DR: In this paper, the authors discuss the occurence of resonances in elastic and inelastic electron-ion collisions and the decay mechanism subsequent to the formation of the intermediate multiply excited state determines whether a resonance is found in recombination, excitation, elastic scattering, in single or even in multiple ionization.
Abstract: The occurence of resonances in elastic and inelastic electron‐ion collisions is discussed. Resonant processes involve excitation of the ion with simultaneous capture of the initially free electron. The decay mechanism subsequent to the formation of the intermediate multiply excited state determines whether a resonance is found in recombination, excitation, elastic scattering, in single or even in multiple ionization. This review concentrates on resonances in the ionization channel. Correlated two‐electron transitions are considered.
3 citations
29 May 2008
TL;DR: In this paper, a single pass merged-beams experiment employing a dense cold electron target recombination of highly charged ions is studied, and unexpected high recombination rates are observed at low energies in the electron ion center-of-mass frame.
Abstract: In a single‐pass merged‐beams experiment employing a dense cold electron target recombination of highly charged ions is studied. Unexpected high recombination rates are observed at low energies Ecm in the electron‐ion center‐of‐mass frame. In particlar, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates at Ecm=0 eV for U28+ and for Au25+ ions. Considerable rate enhancement is also observed for Ar15+. This points to a general phenomenon which has to be interpreted as a consequence of high electron densities, low electron beam temperatures, high ion charge states and presence of strong magnetic fields.
2 citations
12 May 2008
TL;DR: In this paper, the use of heavy ion storage rings and the method of adiabatic magnetic expansion of the electroncooler beam have provided the technical basis for measurements of cross sections and rates with unprecedented quality regarding both statistics and energy resolution Electron ion resonances showing widths as small as 0025 eV have been observed.
Abstract: Recent progress in electron‐impact ionization and recombination of atomic ions in merged‐beams experiments is reviewed The use of heavy‐ion storage rings and the method of adiabatic magnetic expansion of the electron‐cooler beam have provided the technical basis for measurements of cross sections and rates with unprecedented quality regarding both statistics and energy resolution Electron‐ion resonances showing widths as small as 0025 eV have been observed Thus the data provide extremely detailed information about collision processes and atomic structure of ions ranging from He+ to U89+
1 citations
29 May 2008
TL;DR: In this article, the authors present results from measurements of resonant transfer and excitation in collisions of lithium-like F, hydrogen-like S, lithium−like G with H2, and hydrogen−like Ge with Ge with H 2.
Abstract: Recent results on electron capture with simultaneous excitation of a projectile electron (RTE) are reviewed These processes are identified by measuring coincidences between two K‐x‐rays and ion with its charge decreased by one unit, for hydrogen‐like or lithium‐like ions, respectively In the dependence of the cross section on the beam energy it was seen, that this process is resonant and behaves very much like dielectronic recombination In this work we present and discuss results from measurements of resonant transfer and excitation in collisions of lithium‐like F, hydrogen‐like S, lithium‐, and hydrogen‐like Ge with H2 Emphasis is given to investigating the population of very high n states
1 citations
TL;DR: In this paper, the x-ray generating processes in heavy, few-electrons projectiles are summarized within the generalized picture of reduced velocities (adiabaticity), the general energy and Z dependences are shown.
Abstract: In this article, first the x‐ray generating processes in heavy, few‐electrons projectiles are summarized within the generalized picture of reduced velocities (adiabaticity). The general energy and Z dependences are shown. Then, special emphasis is given to RTE (resonant transfer and excitation, see also [1]) as this process is the dominant one in the treated low intermediate velocity region. The advantage of higher resolving capability at heavier projectiles is elucidated. A method to isolate a single RTE resonance, the one to the 2s2p 1P1 level for hydrogenic projectiles, is described. The interesting potentiality to utilize RTE for spectroscopic structure studies of heavy few‐electrons projectiles is demonstrated with the investigation of the 2E1 decay of the 1s2s 1S0 state in He‐like ions. In a last chapter we will change subject and concentrate on multiple‐electron capture yielding to x‐ray emission.
29 May 2008
TL;DR: In this paper, the Bethe-Salpeter approach was used for radiative and dielectronic recombination (RR and DR) of very highly charged ions at energies Ecm=0 to 50 eV.
Abstract: We report measurements on radiative and dielectronic recombination (RR and DR) of very highly charged ions at energies Ecm=0 to 50 eV. Novel techniques were employed at the heavy ion storage ring ESR to obtain absolute recombination rates of Au76+(1s22s). The increase of the RR rate for Ecm→0 could be recorded and single DR resonances in Au75+(1s22p3/26lj) resolved. The experimental data are compared to theoretical results for RR and DR. The calculations for RR are based on the Bethe‐Salpeter approach for completely stripped ions corrected to match Stobbe’s theory and corrected for the presence of several core electrons in the ions studied here. The theoretical RR rates are combined with results for DR from perturbative‐relativistic, semi‐relavistic and fully relativistic theories. There is remarkable agreement between theory and experiment. Nevertheless the high quality of the experimental data allows one to distinguish between the different theoretical approaches to DR.