S-matrix theory of above-threshold ionization in a bichromatic laser field

We apply the strong-field S-matrix theory to the above-threshold ionization (ATI) in a bichromatic linearly polarized laser field having frequencies o mega and 2 omega, and the relative phase phi between the laser field compon ents. The presented theory includes both the Coulomb and rescattering effec ts. We compute and discuss the electron energy spectra for different angles a between the momentum of the ionized electron and the polarization vector of the laser field. We found that the plateau for phi = 0 and for the back ward (theta = 180 degrees) emission of electrons extends up to 21U(p1), whe re U-p1 is the ponderomotive energy of the first laser field component (ass uming equal intensities of both components). There are no such high-energy electrons for theta = 0 degrees, in contrast to the symmetry theta <-> 180 degrees - theta, valid in the monochromatic case. In the bichromatic case t he ionization rates possess the more general symmetry property (phi, theta) <-> (phi + pi. 180 degrees - theta). Therefore, for phi = pi we predict th e emission of the high-energy electrons in the forward direction (theta = 0 degrees). In a bichromatic field the sidelobe structures are strongly infl uenced by quantum mechanical interference effects; We also explore the thet a-dependence of the ionization rates for different relative phases phi, and for those energies which correspond to the classical cutoff law.