Generation of x-ray radiation by atoms in an ultrahigh-intensity laser field

A semiclassical approach is used to describe the generation of high optical harmonics by atoms in an ultrahigh-intensity laser field. Account is taken of the influence of the Coulomb potential and of the relativistic effects on the motion of an electron in a continuum, of the spreading of a wave packet of an electron in a continuum, and also of the depletion of atomic states. A new effect is predicred within a framework of this approach: nonmonotonic behaviour of the intensities of the individual harmonics in a certain range of intensities of the pump field in the form of extremely short laser pulses. It is shown that efficient generation of harmonics with the orders as high as 1000 is feasible.