Relativistic effects in laser-atom interactions at ultra-high intensities

We discuss some of the results of our investigations of relativistic effects in laser-atom interactions at ultra-high intensities. We have solved numerically the time-dependent Dirac equation for a model, one-dimensional atom which is subjected to an ultra-intense, high-frequency laser field. Our method of computation consists of utilizing a B-spline expansion for the wavefunction in momentum space. We demonstrate that for a peak electric field strength of 175 atomic units (a.u.) and for an angular frequency of 1 a.u., relativistic effects are apparent. Even under these extreme conditions the wavefunction remains localized in a superposition of field-free bound states and very-low energy continuum states. Comparing our results with the numerical solution of the time-dependent Schrodinger equation, we find that the Dirac wavefunction is slightly more stable against ionization. It is shown that the relativistic quiver motion of the electron wavepacket differs substantially from the nonrelativistic quiver motion and that the energy distribution of the ionized electrons is strongly concentrated near threshold.