Latest documents http://publikationen.ub.uni-frankfurt.de/index/index/ Tue, 07 Nov 2006 10:10:05 +0200 Tue, 07 Nov 2006 10:10:05 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2669 Using relativistic Green’s-function techniques we examined single-electron excitations from the occupied Dirac sea in the presence of strong external fields. The energies of these excited states are determined taking into account the electron-electron interaction. We also evaluate relativistic transition strengths incorporating retardation, which represents a direct measure of correlation effects. The shifts in excitation energies are computed to be lower than 0.5%, while the correlated transition strengths never deviate by more than 10% from their bare values. A major conclusion is that we found no evidence for collectivity in the electron-positron field around heavy and superheavy nuclei. D. C. Ionescu; Walter Greiner; Berndt Müller; Gerhard Soff article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2669 Tue, 11 Jul 2006 10:10:05 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2430 The energy shift of the 1s1/2 state in 92238U due to virtual excitation of nuclear rotational modes is shown to be a considerable correction for atomic high-precision experiments. In contrast to this, nuclear polarization effects are of minor importance for Lamb-shift studies in 82208Pb. Günter Plunien; Berndt Müller; Walter Greiner; Gerhard Soff article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2430 Fri, 07 Jul 2006 15:24:31 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2425 We consider the contribution of nuclear polarization to the Lamb shift of K- and L-shell electrons in heavy atoms and quasiatoms. Our formal approach is based on the concept of effective photon propagators with nuclear-polarization insertions treating effects of nuclear polarization on the same footing as usual QED radiative corrections. We explicitly derive the modification of the photon propagator for various collective nuclear excitations and calculate the corresponding effective self-energy shift perturbatively. The energy shift of the 1s1/2 state in 92238U due to virtual excitation of nuclear rotational states is shown to be a considerable correction for atomic high-precision experiments. In contrast to this, nuclear-polarization effects are of minor importance for Lamb-shift studies in 82208Pb. Günter Plunien; Berndt Müller; Walter Greiner; Gerhard Soff article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2425 Fri, 07 Jul 2006 12:12:30 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2476 The energy shift of K electrons in heavy atoms due to the self-energy correction has been calculated. This process is treated to all orders in Zα, where Z denotes the nuclear charge. For the superheavy system Z=170, where the K-shell binding energy reaches the pair-production threshold (E1sb∼2mc2), a shift of +11.0 keV is found. This shift is almost cancelled by the vacuum polarization, leaving a negligible effect for all quantum-electrodynamical corrections of order α but all orders of Zα. Gerhard Soff; Paul Schlüter; Berndt Müller; Walter Greiner article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2476 Fri, 30 Jun 2006 12:44:18 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2496 This paper reports calculations of the influence of a reaction time T>10-21 s in deep-inelastic Xe-Pb collisions on the energy spectrum of δ electrons ejected in the same collision. It is shown that the lifetime of the superheavy composite system causes pronounced oscillations of width ε=h/T in the electron distribution, which survive the inclusion of multistep excitations and the folding with a lifetime distribution function. This effect may serve as an atomic clock for deep-inelastic collisions. Gerhard Soff; Joachim Reinhardt; Berndt Müller; Walter Greiner article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2496 Wed, 28 Jun 2006 11:57:22 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2502 The direct (Coulomb) formation of electron vacancies in the 1sσ state of superheavy quasimolecules is investigated for the first time. Its dependence on the impact parameter, projectile energy, and its contribution from excitations into the continum and higher bound states are determined. Wilfried Betz; Gerhard Soff; Berndt Müller; Walter Greiner article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2502 Wed, 28 Jun 2006 11:27:19 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2501 Binding energies and wave functions of inner-shell electronic states in superheavy quasimolecules with (Zp+Zt)α>1 are calculated. Ionization during a collision of very heavy ions is investigated within a molecular basis generated by the solutions of the two-center Dirac equation. Transitions to vacant bound states as well as direct excitation to the continuum are taken into account. We present theoretical values for the ionization probability as a function of impact parameter, bombarding energy, and combined nuclear charge. Our computed results are compared with recent experimental data. It is suggested that relativistic binding energies of electrons in superheavy quasimolecules can be determined experimentally via the impact-parameter dependence of ionization and the anisotropy of quasimolecular radiation. Gerhard Soff; Walter Greiner; Wilfried Betz; Berndt Müller article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2501 Wed, 28 Jun 2006 11:12:35 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2509 The theory of direct electron-positron pair production in the collision of heavy ions is formulated in the framework of the quasimolecular model. The pair production process acquires a collective nature for (Z1+Z2)α>1 and can be understood as the shakeoff of the strong vacuum polarization cloud formed in the quasimolecule. The total cross section is, e.g., 76 μb for Pb + Pb at Coulomb barrier energies. Gerhard Soff; Joachim Reinhardt; Berndt Müller; Walter Greiner article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2509 Wed, 28 Jun 2006 10:20:10 +0200 http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2511 We show that information about quasimolecular electronic binding energies in transient atomic systems of Z=Z1+Z2 up to 184 can be obtained from three sources: (1) the impact-parameter dependence of the ionization probability; (2) the ionization probability in head-on collisions as a function of total nuclear charge Z; (3) the delta-electron spectrum in coincidence with K-vacancy formation in asymmetric collisions. Experiments are proposed and discussed. Gerhard Soff; Berndt Müller; Walter Greiner article http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/2511 Wed, 28 Jun 2006 09:32:37 +0200