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Quantum electrodynamics of strong external fields is investigated in the context of atomic physics. If the electromagnetic coupling constant Z a becomes sufficiently large (Z > l / a % 137 for point sources or Z > 172 for extended nuclei) bound electron states can join the negative-energy continuum of the Dirac equation. The resulting possibility of spontaneous positron production and the new concept of a charged electron-positron vacuum is discussed under several aspects. The autoionization model and the exact overcritical solutions of the single-particle Dirac equation are contrasted with a quantum-field-theoretical approach. Non-linear field effects are shown to have no influence. Vacuum polarization and the self-screening of extremely strong electric charges are treated explicitly. Adiabatic collisions of heavy ions leading to the transient formation of quasimolecular electron orbitals are suggested as a means for an experimental test of the special features of strong-field QED, After an introduction to the relativistic two-centre Dirac equation, the x-ray spectroscopy of quasimolecules and the spontaneous and induced positron emission in, for example, U-U collisions are discussed. The importance of several background processes is stressed. Finally, strong-field aspects for nuclear matter and in gravitation are mentioned.

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Collisions of very heavy ions at energies close to the Coulomb barrier are discussed as a unique tool to study the behavior of the electron-positron field in the presence of strong extemal electromagnetic fields. To calculate the excitation processes induced by the collision dynamics, a semiclassical model is employed and adapted to describe the field-theoretical many-particle system. An expansion in the adiabatic molecular basis is chosen. Energies and matrix elements are calculated using the monopole approximation. In a supercritical (2, + Z, 2 173) quasiatomic system the 1s level joins the antiparticle continuum and becomes a resonance, rendering the neutral vacuum state unstable. Several methods of treating the corresponding time-dependent problem are discussed. A projectionOperator technique is introduced for a fully dynamical treatment of the resonance. Positron excitation rates in s", andp,,] states are obtained by numencal solution of the coupled-channel equations and are compared with results from first-plus second-order perturbation theory. Calculations are performed for subcritical and supercritical collisions of Pb-Pb, Pb-U, U-U, and U-Cf. Strong relativistic deformations of the wave functions and the growing contributions from inner-shell bound states lead to a very steep Z dependence of positron production. The results are compared with available data from experiments done at GSI. Correlations between electrons and positrons are briefly discussed.