Relativistic effects in laser-atom interactions

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 mc(2) becomes comparable to unity. this rtio is given by q = e(2)E(0)(2)/(4m(2)omega (2)c(2)), where E-0 is the peak strength of the electric field, omega is the angular frequency of the laser light and e is the magnitude of the electron charge. 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 is paid to the modifications of the laser-atom dynamics in the high-intensity, high-frequency regime.