The results of a study on vacuum polarization, orders $\alpha {\left(Z\alpha \right)}^n$, $n>~3$, for large-$Z$ systems encountered in heavy-ion collisions are presented. It is shown that the higher-order vacuum polarization cannot prevent the $1S_{\frac{1}{2}}$ state from reaching the lower continuum, $E=-m_e{c}^2$, for some critical charge $Z_{\mathrm{cr}}\sim 170$. In addition, the stability and localization of a heliumlike system for $Z>\mathrm{}{Z}_{\mathrm{cr}}$ is demonstrated.

• Received 5 August 1974

DOI:https://doi.org/10.1103/PhysRevLett.33.921