FIG. 2: Schematic view of the electron energy levels in a strong electrostatic potential − V ( z ). The electron states above (Fermi sea) and below (Dirac sea) the mass gap (white area) are shown by dark and grey shadowing. Transitions from the Dirac to Fermi sea (long arrow) are not allowed because all states in the Fermi sea are occupied. production of negative muons and pions, which may replace the electrons. They become possible in a strong enough field when V$_{0}$ > m$_{µ}$, m$_{π}$ . This opens the possibility of pion condensation induced by strong electric fields [17]. With increasing a the electron density comes closer and closer to the proton density and the charge separation at the surface diminishes. The electric potential difference at z → ±∞ is fixed by the boundary conditions, but its gradient, i. e. the electric field, across the transition region becomes smaller and smaller. We have checked that the electric field falls below the critical value at a ≈ 10 pm. In Table 1 we give the maximum value of the electric field, E$_{max}$, calculated for several values of the diffuseness parameter a . E$_{max}$ can be 6