A rigorous evaluation of the time evolution amplitude
G(t) for an isolated autoionizing state has yielded the result ${{|G(t)|}^{2}}_{t\ensuremath{\rightarrow}\ensuremath{\infty}}={e}^{\ensuremath{-}\frac{\ensuremath{\Gamma}t}{\ensuremath{\hbar}}+\frac{{\ensuremath{\hbar}}^{2}}{{\ensuremath{\pi}}^{2}(4{E}^{2}+{\ensuremath{\Gamma}}^{2}){t}^{2}}}$ where
E is the energy above the ionization threshold and $\ensuremath{\Gamma}$ the width. For states with lifetimes in the region ${10}^{\ensuremath{-}8}$-${10}^{\ensuremath{-}11}$ sec near the ionization threshold, the nonexponential decay corrections are sufficiently large to suggest that perhaps it is possible to test the validity of the fundamental exponential "law" within the realm of atomic physics.
We demonstrate that the classical helium atom exhibits algebraic decay of the survival probability in the autoionizing
eZe configuration. Excitation towards asymptotic orbits of marginal stability is identified as the cause of the nonexponential decay. Since one electron travels along a bound Kepler trajectory of infinite excitation in these orbits, we argue that the classical decay law will prevail in the quantum dynamics of a wave packet launched along the
eZe configuration.