Veranstaltungskalender

Atomphysik-Seminar

• Mittwoch, 30. Mai 2012   11.00 Uhr -- Vorraum Atomphysik, Südbau, II. Stock
  Dietrich Bernhardt, Univ. Giessen
  "Dielectronic recombination of lithium- and berylliumlike xenon ions"
  

  In recent years dielectronic recombination was developed from the study of the recombination process itself to a tool which allows to study plasma rate coefficients, excitation energies, isotope effects and transition rates of highly charged ions. Here, experimental results for the dielectronic recombination (DR) of lithium- and berylliumlike xenon ions are presented. Absolute rate coefficients for DR have been measured at the heavy-ion storage ring ESR. For the measurement the electron cooler was alternatingly used as an electron target and for ion-beam cooling. The investigated center-of-mass energy range 0 - 550 eV covers all DR resonances belonging to the intra-L-shell excitations. Measurements are compared with multiconfiguration Dirac-Fock calculations. Intra-L-shell excitation energies are extracted from the measured DR spectra and are compared with calculations and spectroscopic results. For berylliumlike xenon in addition to the prominent ground-state DR resonances, DR associated with metastable 2s2p 3P0 parent ions was also observed. For the investigated xenon isotope no single-photon deexcitations of this state are allowed. Thus, this state is long-lived with a E1M1 two-photon transition to the ground state being its dominant decay channel. The depopulation of the metastable beamfraction is observed and compared with theoretical predictions for the E1M1 two-photon-transitions. DR resonance spectroscopy seems to be well suited for further studies of E1M1 two-photon-transitions in heavy berylliumlike ions.
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• Mittwoch, 06. Juni 2012   11.00 Uhr -- Vorraum Atomphysik, Südbau, II. Stock
  Stanislav Tashenov, Univ. Heidelberg
  "Spin dynamics of electrons in strong fields studied via bremsstrahlung from a polarized electron beam"
  

  Linear polarization of hard x-rays emitted in the process of the atomic field electron bremsstrahlung has been measured with a polarized electron beam. The correlation between the initial orientation of the electron spin and the angle of photon polarization has been systematically studied by means of Compton and Rayleigh polarimetry techniques applied to a segmented germanium detector. The results are in a good agreement with the fully-relativistic calculations. They are also explained classically and in a unique way manifest that due to the spin-orbital interaction the electron scattering trajectory is not confined to a single scattering plane. The developed photon polarimetry technique with a passive scatterer is very efficient and accurate and thus allows for novel applications. Bremsstrahlung polarization correlations lead to a new method of polarimetry of electron beams. Such a method is sensitive to all three components of the electron spin. It can be applied in a broad range of the electron beam energies from ≈100 keV up to a few 10 MeV. The results of the test measurement at 100 keV will be shown. The optimum scheme for electron polarimetry will be analyzed and the relevant theoretical predictions will be presented.
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• Mittwoch, 18. Juli 2012   11.00 Uhr -- Vorraum Atomphysik, Südbau, II. Stock
  Thomas Pfeifer, MPI-K, Heidelberg
  "Shifting paradigms in ultrafast atomic and molecular physics: Precision spectroscopy of 2-electron dynamics with attosecond pulses and noise-enhanced temporal resolution at FELs"
  

  Two parallel revolutions in laser science have recently led to the generation of attosecond pulsed light on one hand and intense x-ray light on the other hand. Despite this progress on the table-top optical and free-electron laser (FEL) light-source side, respectively, direct experimental access to time and space-resolved quantum motion, in particular to the full wave function of two or multiple electrons remained a formidable challenge. One major focus of my research and topic of this talk is the development of a universal ultrafast spectroscopy and imaging toolkit, designed for these novel light sources, to gain access to intra-atomic and -molecular motion. Combination of attosecond technology with high-resolution spectroscopy recently allowed us to measure near-valence wave-packet motion of two excited electrons in helium oscillating with a period of 1.2 fs. We thereby discovered a general control mechanism acting on the phase of two-electron wave functions, an essential ingredient in the laser synthesis of chemical bonds, typically featuring two electrons per binding orbital. We also recently developed the conceptual framework of enhancing temporal resolution in pump–probe experiments by using correlated noise existing e.g. in FEL pulses. The time-dependent molecular wave function in D2 molecules can thus be probed with few-femtosecond temporal and sub-Angstrom spatial resolution, even though the duration of the laser pulses (~30 fs) and their wavelength (33 nm) are order(s) of magnitude larger. Using coincidence detection methods (Reaction microscopes aka COLTRIMS) in tandem with the novel laser sources, the multiple-ionization dynamics of I2 was measured, also resolving its dissociation. This allows us to smoothly observe the transition from a molecule to an atom interacting with strong high-frequency FEL pulses, of interest for future applications of x-ray FELs towards the imaging of structure and dynamics in larger molecules of biological interest.
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