Stabilization of an atomic system in a strong laser field and the Kramers-Henneberger approach

Ionization of a quantum system with a short-range potential by a linearly p olarized field is numerically studied within a wide range of laser intensit ies and frequencies. The dynamics of such a system is interpreted in terms of both Kramers-Henneberger eigenstates and field-free atomic states. The e xact solution is analyzed to determine which potential (atomic or Kramers-H enneberger potential) provides the most adequate physical description of th e ionization of the system. The validity of the Kramers-Henneberger approac h is examined, in particular, for low laser frequencies. In addition to the condition obtained by Gavrila and Kaminski, we found another, stronger lim itation on the validity range of the Kramers-Henneberger approximation. Low -frequency stabilization is shown to occur only if the Kramers-Henneberger approach is physically correct and is applied within its validity range.