Relativistic effects in laser-atom interactions

C. J. Joachain
Physique Théortique, Université Libre de Bruxelles, Belgium
and
N. J. Kylstra
Department of Physics, University of Durham, UK

Relativistic effects arise when atoms interact with ultrastrong laser fields. Such effects are expected to become important when the ratio of the ponderomotive energy U_p of an electron in the field to the electron rest mass energy becomes comparable to unity. In atomic units, this ratio is U_p/c^2=E_0^2/(4 w^2 c^2), where E_0 is the peak strength of the electric field, w is the angular frequency of the laser light and c is the velocity of light. We discuss recent progress in the theoretical study of several relativistic effects in laser-atom interactions: influence of the magnetic component of the laser field, relativistic mass shift due to the dressing of the electron mass by the laser field, negative energy states and spin effects. Particular attention will be paid to the modifications of the laser-atom dynamics in the high-intensity, high-frequency regime where non-relativistic theories predict a decrease of the ionization probability of atoms with increasing laser intensity.