TUNNEL ABOVE-THRESHOLD IONIZATION - ELECTRON RESCATTERING AND GENERATION OF SHORT-WAVELENGTH RADIATION

A semiclassical approach is developed to investigate above-threshold i onization of atoms in a high-intensity laser field. The influence of t he Coulomb potential of a parent ion and relativistic effects on elect ron motion in the continuum and electron wave packet spreading were ta ken into account. Numerical simulations show that these factors lead t o a modification of the cut-off frequency law from the well-known form ula [1] Omega = 3.17U(p) + I-p to Omega = f(I)U-p + I-p (atomic units e = m = (h) over bar = 1 are used throughout this paper), where U-p is the electron ponderomotive energy and I-p is the atom ionization pote ntial. It is shown that a maximum cut-off frequency Omega(max) should exist. The critical laser intensity corresponding to this frequency is I-cr similar to omega(2), where omega is the laser frequency. It is s hown that the electron energy and angular distributions are significan tly modified. Also, an explanation of processes resulting in the forma tion of the region behind the cut-off of high-harmonic spectra is pres ented.