Applicability of the Kramers-Henneberger approximation in the theory of strong-field ionization

The ionization of the one-dimensional model system with a short-range poten tial by a linearly polarized laser field is studied by direct numerical int egration of the non-stationary Schrodinger equation. The results are interp reted in terms of both field-free atomic states and Kramers-Henneberger (KH ) eigenstates. The dynamics of the system is analysed to determine which po tential (atomic or KH) is physically more appropriate to describe the ioniz ation process. The justification of the KH approach is also examined. A new limitation of the validity of the KH approximation is found to exist in th e case of low laser frequencies.