Volkov limit for atomic ionization by ultraintense laser fields

We examine the Volkov limit in the interaction of an atom with an ultrastrong laser field, where the atom-laser coupling energy becomes larger than the atomic binding energy. The full time-dependent wave function is expressible as an expansion in the atomic potential. Numerical results are obtained for the ionization probability of a one-dimensional {δ}-potential model atom, for which full dynamical solutions are also found. We find uniform convergence toward full ionization with increasing field strength, with no sign of ground-state stabilization.