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In atomic systems with high nuclear charges, inner-shell electrons experience extremely high nuclear binding fields. The dynamics of electrons becomes relativistic and also the electronic probability density has a considerate overlap with the nuclear matter. These properties render highly charged ions an ideal tool for basic research in relativistic atomic structure theory, quantum electrodynamics at extreme fields, and nuclear size contributions. Accurate knowledge of the properties of highly charged ions is also necessary for the diagnostics of hot fusion plasmas and astrophysical plasmas. In few-electron ions, relativistic and nuclear effects are intertwined with large electron correlation contributions. Because of this complex interplay, accurate theoretical calculations are necessary to explain the physical phenomena observed in increasingly accurate experimental spectra.
We also investigate energetic collision and recombination processes of highly charged ions with electrons and atoms. Projects have also been started into the interaction of intense lasers in the optical as well as the x-ray regime with such relativistic ions, and in the field of quantum electrodynamic and nuclear effects on the bound electron g factor. Furthermore, in recent theoretical simulations we study the acceleration of protons and heavier ions by strong laser fields.
We perform theoretical investigations concerning the strong-field dynamics of ions in order to develop optimal schemes for laser ion acceleration. Laser-accelerated beams may find applications in industry, basic research and in medicine. As an example, cancer hadron therapy requires accelerated ion beams of high energy sharpness and a narrow spatial profile in order to accurately irradiate the tumor while sparing damage from the surrounding healthy tissue. As we have shown in theoretical simulations [1], linearly and radially polarized, tightly focused and thus extremely strong laser beams should permit the direct acceleration of light atomic nuclei up to energies that may offer the potentiality for medical applications and may allow one to build more economic cancer treatment facilities. In a different setting employing two identical crossed laser beams [2], ions originating from a laser-plasma process can be efficiently post-accelerated to form a beam of high intensity, energy and quality.
Figure 1 (left): Ion trajectories (blue) calculated by a simulation of the acceleration in the focus of a radially polarized laser beam. The red arrows illustrate the radial and longitudinal field vectors and the propagation direction of the beam. (right): Post-acceleration of an ion beam generated in a laser-plasma interaction process by two crossed lasers.
The photoionization (PI) of atomic systems may proceed via a resonant two-step mechanism: In the first step, a photon is absorbed by resonant excitation to an autoionizing intermediate state embedded into the electronic continuum (see Fig. 2.). In the second step, the so-formed state autoionizes. In the case of highly charged ions (HCIs), the photon energies needed are in the x-ray (short-wavelength) domain. In astrophysics, PI of HCIs has been recently observed in high-resolution x-ray spectra of quasars, which display absorption lines due to the presence of HCIs in the line of sight. Such data have revealed the hitherto only presumed existence of a tenuous warm-hot intergalactic medium, the so-called WHIM, comprising a large fraction of baryonic matter. For the identification and analysis of absorption lines due to HCIs in astrophysical spectra, a large amount of reliable theoretical or experimental PI data is needed. We perform fully relativistic distorted wave calculations of PI cross sections, accompanying recent measurements employing an electron beam ion trap (EBIT) and a synchrotron as a source of x-ray photons in the keV range [1,2].
Figure 2: Resonant photoionization illustrated for the case of a four-electron ion. The electronic shell is resonantly excited by the x-ray photon to an autoionizing intermediate state, which subsequently decays by a correlated Auger process.
| [1] M. C. Simon, J. R. Crespo López-Urrutia, C. Beilmann, M. Schwarz, Z. Harman, et al., Phys. Rev. Lett. 105, 183001 (2010) (see also: "Der letzte Schrei aus dem Schwarzen Loch", Press information by the Max Planck Society (in German); "Physicists produce black hole plasma in the lab", Physorg.com) |
| [2] M. C. Simon, M. Schwarz, S. W. Epp, C. Beilmann, B. L. Schmitt, et al., J. Phys. B: At. Mol. Opt. Phys. 43, 065003 (2010) |
In the process of radiative recombination (RR), a photon is directly emitted by an electron, i.e. it is the time-reverse of the photoelectric effect. Alternatively, in a two-step process, the incoming electron resonantly excites a bound electron during recombination, leading to dielectronic recombination (DR, see Fig. 3). Furthermore, in some ionic species, the simultaneous excitation of two bound electron may occur. This higher-order process may be termed as trielectronic recombination (TR). These photorecombination processes involving highly charged ions in collisions with energetic electrons are important for a number of applications. In particular, DR is known as a source of the severe energy losses from high temperature plasmas such as magnetically confined fusion plasmas or stellar plasmas, and, thus, precise quantitative understanding of such phenomena is indispensable. We calculate cross sections and accurate resonance peak positions. These calculations are tested by high-resolution electron beam ion trap (EBIT) (see [1,2,3]) and storage ring measurements.
Figure 3: Level schemes illustrating the resonant processes of di-, tri- and quadrelectronic recombination in an initially carbonlike (six-electron) ion.
In the presence of a neighboring atom, electron-ion recombination can proceed resonantly via excitation of an electron in the atom, with subsequent relaxation through radiative decay, as illustrated on Fig. 4. In joint collaboration with the groups of Carsten Müller and Alexander B. Voitkiv it has been shown that this two-center dielectronic recombination (2CDR) process can largely dominate over single-center radiative recombination at internuclear distances as large as several nanometers [4]. It may play a significant role in dense plasma and chemical environments where it can substantially affect the quantum dynamics and time evolution of the system.
Figure 4: Level scheme depicting the two-center dielectronic recombination process. The electron recombining radiationlessly with center A transfers its energy to resonantly excite center B. (Figure from Ref. [4], copyright of the American Physical Society.)
Isotope shift measurements of dielectronic recombination cross sections performed in storage rings, combined with our atomic structure calculations, allow one to extract information on the nuclear charge distribution of the isotopes involved and thus about the interplay of forces that act between the nuclear constituents. In a theoretical study, we analyze the dependence of electron interaction and QED contributions of the bound-state electronic transition energy on the nuclear size in order to determine the change of nuclear radii corresponding to the isotope shift in three-electron neodymium. Experimental isotope shifts have been measured with the ESR storage ring of the GSI Darmstadt [1]. This approach, based on a close collaboration of theory [2] and experiment, constitutes a new technique to determine nuclear charge radii.
Figure 5: The change of nuclear size or mass causes an isotope shift of the observed spectrum
The high-precision theory of multiply charged ions has been investigated in EBIT group of our institute. Extensive theoretical studies of the ground-state forbidden magnetic dipole transition in boronlike (five-electron) argon ions have been performed. The mass isotope shift associated with this line have been determined with high accuracy using the EBIT. The wavelengths of the above transition were compared for the isotopes 36Ar and 40Ar. The observed isotope shift has confirmed the relativistic theory of nuclear motional effects in many-body systems [3]. Calculations based on the fully relativistic recoil operator are in excellent agreement with the measured results.