Deutsche Physikalische Gesellschaft e. V. (DPG)

WWW-Server für Frühjahrstagungen und E-Verhandlungen

E-Verhandlungen 2001
Programm und Abstracts der Sitzung A 19

Absolute electron impact ionization cross sections of Ne, Ar, Kr, and Xe

•Mathias Richter¹, G. Ulm¹, A.A. Sorokin², L.A. Shmaenok² und S.V. Bobashev^2
1Physikalisch-Technische Bundesanstalt, Abbestrasse 2-12, 10587 Berlin, Germany
²Ioffe Physico-Technical Institute RAS, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia

A method and apparatus for precise measurements of absolute total cross-sections for electron-impact ionization have been developed. The method is based on the comparison of the total ion yields resulting from ionization of rare gases by electrons and photons. Ratios of total cross sections for electron-impact ionization and photoionization in Ne, Ar, Kr, and Xe in the energy range from 140 eV to 4000 eV for electrons and from 16 eV to 1012 eV for photons were measured. On the basis of our measured cross-section ratios and well-known total photoionization cross section data, we deduced absolute total electron-impact ionization cross sections for Ne, Ar, Kr, and Xe with relative standard uncertainties as low as 0.03. A comparison of the cross-section data with published experimental and theoretical results is given.

A.A. Sorokin et al., Rev. A 58 (1998) 2900; Phys. Rev. A 61 (2000) 022723.

Recoil spectroscopy with laser-cooled trapped Na-atoms

•J.W. Turkstra¹, R. Hoekstra¹, S. Knoop¹, D. Meyer² und R. Morgenstern^1
1KVI, Rijksuniversiteit Groningen, Zernikelaan 25, NL-9747 AA Groningen, The Netherlands
²Experimentalphysik insbes. Gaselektronik, Ruhr-Universität Bochum, D-44780 Bochum, Germany

We have studied electron transfer processes during collisions of highly charged ions on an atomic Na target. To this end the Na atoms are cooled and trapped in a magneto optical trap (MOT) by means of laser radiation. Position and arrival time of the Na^q+ ions resulting from the collisions are measured with a position sensitive detector, and from this the corresponding recoil momenta are deduced. Momentum transfer perpendicular to the O⁶⁺, C⁶⁺ and He²⁺ projectile beam direction increases drastically with increasing number of transferred electrons. We observe transfer of up to 3 electrons for He²⁺ and up to 4 electrons for O⁶⁺ and C⁶⁺. We compare the experimental results with predictions of the classical over-the-barrier model and with classical trajectory Monte Carlo calculations.

Low-energy electron ion recombination in a magnetic field

•Maria Hörndl, Shuhei Yoshida, Karoly Tokesi und Joachim Burgdörfer
Institute for Theoretical Physics, Vienna University of Technology Wiedner Hauptstr. 8-10, A-1040 Vienna, Austria

The electron-ion recombination in cold magnetized plasmas is currently intensively studied both experimentally and theoretically. Recent storage ring experiments show a dramatic enhancement of the radiative recombination rate for high Z ions relative to what standard radiative recombination rates predict. We analyze the role of classical chaotic dynamics and unstable periodic orbits which bring an electron to multiple visits to the target ion and enhance the recombination. In addition, a role of stochastic perturbations in the dilute electron gas will be considered. This work is supported by FWF and SFB016.

Examinations of KLL capture of electrons into highly charged heavy ions

•Z. Harman, S. Zakowicz, N. Grün und W. Scheid
Institut für Theoretische Physik der Justus-Liebig-Universität Gießen

Relativistic calculations for RTE processes (Resonant Transfer and Excitation) are presented for the collisions of hydrogen-like uranium ions with hydrogen atoms in the energy range of the KLL resonances. First the cross sections for dielectronic recombination (DR) are obtained perturbatively within the isolated resonances approximation. The RTE cross sections are given by a folding of the DR cross sectios with the Compton profile of the target atoms. The influence of higher order terms of the electron-electron interaction going beyond the Breit interaction will be discussed. The possible effects originating from the emission of more than one photon will also be covered.

Rate enhancement in the recombination of fully stripped U⁹²⁺ and Bi⁸³⁺ ions with electrons

•W. Shi¹, T. Bartsch¹, S. Böhm¹, C. Böhme¹, C. Brandau¹, A. Hoffknecht¹, S. Kieslich¹, H. Knopp¹, S. Schippers¹, A. Müller¹, C. Kozhuharov², F. Bosch², B. Franzke², P.H. Mokler², M. Steck², Th. Stöhlker² und Z. Stachura^3
1Institut für Kernphysik, Universität Giessen, D-35392 Giessen, Germany
²Gesellschaft für Schwerionenforschung (GSI), D-64291 Darmstadt, Germany
³Institute for Nuclear Physics, 31-342 Kraków, Poland

Recombination of U⁹²⁺ and Bi⁸³⁺ ions with electrons has been investigated at the Experimental Storage Ring (ESR). Absolute recombination rate coefficients have been measured for relative energies from 0 up to 125 eV. For energies greater than about 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. This so called "low-energy rate enhancement", observed previously in the recombination of light ions, has been investigated for the first time in the recombination of the heaviest ions. The rate enhancement is shown to scale as Z^2.6 for bare ions. The recombination rate enhancement is insensitive to changes of the electron density. Variation of the magnetic guiding field strength from 70 mT to 150 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.

Potential scattering of wave packets

•Mikhail Fedorov
General Physics Institute, Russian Academy of Sciences, 38 Vavilov st., Moscow 117942 Russia

Scattering of electrons by a ponderomotive potential of a focused laser light is considered. Initial electron wave function is taken in the form of a wave packet. Non-stationary Schroedinger equation is solved in the approximation of small spreading of a wave packet during the electron-target interaction time. Similarity to and differences with a standard stationary theory of plane-wave scattering is discussed, as well as the conditions of a transition to the classical scattering picture.

QED corrections to the REC in heavy highly charged ions

•Thomas Beier¹, Jörg Eichler², Vladimir M. Shabaev^2,3 und Vladimir A. Yerokhin^2,3
1GSI, Atomphysik, Planckstr. 1, D-64291 Darmstadt, Germany
²HMI, Theoretische Physik, Glienicker Str. 100, D-14109 Berlin, Germany
³St. Petersburg State University, Dept. of Physics, 198904 St. Petersburg, Russia

We present our calculations of the quantum electrodynamical (QED) corrections to the radiative electron capture in heavy highly charged ions. Formulae for all corrections of order a were derived in the framework of the two-time Green function method, and the calculations were carried out employing propagators for electrons in the Coulomb field of the nucleus [1]. Therefore the calculations are fully relativistically and consider the binding to all orders. Results for the differential cross sections are presented which amount up to 1 % for uranium projectiles with an energy higher than 1 GeV/u. As a next step, we are going to investigate the pair production related to this process.
The work is supported by the DFG (Grants No. 436 RUS 113/479 and 113/616), the RFBR (Grant No. 98-02004111), by GSI, and by the EU-TMR programme (ERB FMRX CT 97-0144).
[1] V.M. Shabaev et al., Phys. Rev. A 61 (2000) 052112

High Precision Measurement of the g_j-factor of the electron bound in hydrogen-like oxygen for testing the Quantum Electrodynamics

•J. Verdú¹, H. Häffner², N. Hermanspahn¹, T. Valenzuela¹, S. Djekic¹, H.-J. Kluge², W. Quint², S. Stahl² und G. Werth^1
1Universität Mainz, 55099 Mainz
²GSI Darmstadt, Planckstr. 1, 64291 Darmstadt

Text, genau einmal We report on a new measurement of the g_j-factor of the electron bound in hydrogen-like oxygen. Our preliminary result is g_j = 2,000  047  017  (5). The experiment was performed on a single ¹⁶O⁷⁺ ion in a double Penning trap. Our result , with a relative accuracy of 2 ·10^-9 , allows the test of performed calculations on bound state QED effects with very high precision. In addition , it opens the possibility of determining fundamental quantities like the electron mass or the fine structure constant a with improved precision compared to currently accepted values. Single particles in a trap represent an almost ideal system for testing physical theories ; for instance, the QED in weak fields could be impressively confirmed through the measurement of the g-factor of the free electron ( Dg/g = 10^-11).The g_j-factor of the bound electron is also affected by additional relativistic and QED-corrections due to the strong electric field of the nucleus. In the present experiment , we prepare and store a single highly charged ion in a Penning trap. The ion is detected and cooled down at 4 K by means of a superconducting tank circuit. For determining the g_j-factor , the Larmor and cyclotron frequencies are measured with very high accuracy. The spinflip transitions are observed non-destructively with the use of a strong magnetic-bottle.

Theory for the g factor of the electron in hydrogenlike carbon and a new value for the electron mass

•Thomas Beier¹, Savely G. Karshenboim², Hartmut Häffner^1,3, Nikolaus Hermanspahn³, H.-Jürgen Kluge¹, Gerrit Marx¹, Wolfgang Quint¹, Tristan Valenzuela³, José Verdú³ und Günther Werth^3
2D.I.Mendeleev Institute for Metrology, St. Petersburg, 198005, Russia
³Institut für Physik, Universität Mainz, 55099 Mainz, Germany
¹GSI, 64291 Darmstadt, Germany

The g factor of an electron bound in hydrogenlike carbon was measured to be 2.001 041 596 4 (8)(6)(44) where the uncertainties are due to statistical and systematical errors and the current value of the electron mass [1]. This value is in agreement with theoretical values of nearly the same precision [2,3] which in turn allows the determination of an independent value for the electron mass from this most precise comparison of quantum electrodynamical theory and experiment in any multiply charged system. We will elucidate the theoretical contributions which form the basis of this new mass determination.
[1] H. Häffner et al., PRL 85 (2000) 5308.
[2] S. G. Karshenboim, Phys. Lett. A 266 (2000) 380.
[3] T. Beier et al., Phys. Rev. 62 (2000) 032510.

Electron-positron pair production in relativistic collisions of heavy ions

•M. Gail, N. Grün und W. Scheid
Justus-Liebig-Universität Giessen

In collisions of heavy ions electron-positron pairs can be produced by the strong electromagnetic fields of the colliding ions. One distinguishes free pair production from pair production with capture of the electron into a bound state of an ion (bound-free pair production). For very small impact parameters perturbation theory becomes invalid and nonperturbative methods must be used for the proper description of the collision process. We use the coupled channel approximation with a basis set consisting of atomic wave functions centered around the projectile and the target nucleus. The continuum is discretized by employing wave packets. We have calculated cross sections for bound-free and free pair production in collisions of U⁹²⁺ ions and La⁵⁷⁺ ions with Au, Ag and Cu at a collision energy of 960 MeV/nucleon. We compare our results with experiments that have been performed by Belkacem et al. [1] at the BEVALAC. Wirkungsquerschnitten

[1] A. Belkacem, H. Gould, B. Feinberg, R. Bossingham and W. E. Meyerhof, Phys. Rev. A 56, 2807 (1997)

Measurement of Superelastically Scattered Electrons in Metastable O⁶⁺, C⁴⁺ + H₂ Collisions

•A. L. Landers¹, A.S. Al-Naser¹, H. Knutson², D.J. Pole¹, T. W. Gorczyca ¹ und J.A. Tanis^1
1Western Michigan University, Kalamazoo, Michigan, USA 49008
²Kalamazoo Area Math and Science Center, Kalamazoo, Michigan, USA 49008

We have measured the scattering of quasi-free target electrons by metastable He-like oxygen ions. The objective is to study the mechanism by which an excited ion can transfer its energy to a scattering electron. In this case, the reaction is simply a temporally reversed inelastic excitation process. We have measured electrons scattered by the 1s2s ³S metastable component of a 1.1 MeV/u O⁶⁺ and 0.6 MeV/u C⁴⁺ ions produced by the Western Michigan University tandem Van de Graaff accelerator. A loosely bound electron from the hydrogen target scatters from the projectile ion at zero degrees in the laboratory frame. Electrons which gain energy from the de-excitation of a metastable projectile manifest themselves in a broad peak in the electron spectrum consistent with the energy gain from the de-excitation of the metastable ion. This reaction is under further investigation encompassing other ion species and energies and comparing these results to R-matrix electron scattering calculations.

Electron Scattering by Xe at Intermediate and High Energies: 0.5 - 10 keV

•Gustavo Garcia¹, Jose Luis de Pablos¹, Francisco Blanco² und Amalia Williart^3
1CIEMAT,Avenida Complutense 22,Edificio 2,28040 Madrid,Spain
²Universidad Complutense de Madrid,Avenida Complutense sn,28040 Madrid,Spain
³UNED,Senda del Rey 9,28040 Madrid,Spain

Important applications based on energy deposition models of radiation in matter require cross section data for electron scattering by atoms and molecules over a wide energy range. However reliable data for both elastic and inelastic processes at electron energies ranging from 0.5 to 10 keV are scarce, and especially for heavy atoms as Xe. Moreover, recent total cross section measurements for atomic and molecular targets show significant discrepancies for energies above 1 keV. In this study, the total cross sections for electron scattering by Xe have been measured for energies ranging from 0.5 to 5 keV by using a transmission-beam technique. A detailed analysis of possible systematic errors is also given. Differential and integral elastic cross section have been calculated in the energy range 0.5-10 keV by using a scattering potential method. Finally, integral inelastic cross section have been obtained by combining the total cross sections with the corresponding integral elastic ones.

[HOME]

[Archiv]

[E-Verhandlungen 2001]

[Berlin]

[Fachverband A]

[Sitzung A 19]

Zuletzt geändert am 05.06.2001

Fragen, Kommentare, Anregungen und Kritik