Intensity dependence of plateau structures in laser-assisted x-ray-atom scattering processes

The dependence on laser intensity of recently discovered plateau structures in laser-assisted x-ray-atom scattering, both with and without a static el ectric field present, is analyzed. Using the "three-step" model and the str ong-field approximation we demonstrate a connection between laser-assisted, x-ray-atom scattering and high-order harmonic generation: For high laser-f ield intensities without a static field present, both processes have platea us whose energies extend to the cutoff value 3.17U(p), where U-p is the pon deromotive potential energy. For x-ray-atom scattering in the presence of a static electric field we show that at high laser-field intensities two pla teaus appear: One is our recently predicted high-energy plateau for the sam e process [Phys. Rev. Lett. 81, 5097 (1998)], while the other, low-energy p lateau, has a differential cross section six orders of magnitude larger. Th e energy positions and relative magnitudes of these new plateaus are explai ned using semiclassical arguments. [S1050-2947(99)07211-X].