RELATIVISTIC EFFECTS IN THE TIME EVOLUTION OF A ONE-DIMENSIONAL MODELATOM IN AN INTENSE LASER FIELD

Using a B-spline expansion in momentum space, we have solved the time- dependent Dirac equation numerically for a model, one-dimensional atom which is subjected to an ultra-intense, high-frequency laser field. W e find that for a peak electric field strength of 175 atomic units (au ) and for angular frequencies of I and 2 au, relativistic effects star t to become apparent. Even under these extreme conditions the wavefunc tion remains localized in a superposition of field-free bound states a nd very-low-energy continuum states. Comparing our results with the nu merical solution of the time-dependent Schrodinger equation, we find t hat the Dirac wavefunction is slightly more stable against ionization. We also find that the energy distribution of the ionized electrons is strongly concentrated near threshold and that a cut-off in the high-e nergy spectrum occurs at the energy corresponding to the maximum momen tum of a classical electron in the laser field.