Electron shells in over-critical external fields

When the charge of a nucleus exceedsZ=Z _cr≈ 164–172 the energy of the 1s-electron level is lowered beyond the critical value of ?m _e c ². Then this bound level is degenerated with negative energy continuum solutions of the Dirac equation and becomes a resonance, whose shape varies and is approximately of Breit-Wigner type forZ?Z _cr?5. The physical meaning of this resonance can be understood most easily if the 1s-level is unoccupied (K-hole). In this case a positron may escape ifZ>Z _cr, a process, that can be interpreted as auto-ionization of the positron. This fundamentally new process of quantum electrodynamics of strong fields can be tested experimentally by scattering very heavy ions (Z≧80) on each other since in such collisions superheavy electronic molecules occur (superheavy quasimolecules).