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 ω and 2ω, and the relative phase φ between the laser field components. The presented theory includes both the Coulomb and rescattering effects. We compute and discuss the electron energy spectra for different angles θ between the momentum of the ionized electron and the polarization vector of the laser field. We found that the plateau for φ = 0 and for the backward (θ = 180°) emission of electrons extends up to 21Up1, where Up1 is the ponderomotive energy of the first laser field component (assuming equal intensities of both components). There are no such high-energy electrons for θ = 0°, in contrast to the symmetry θ ↔ 180° - θ, valid in the monochromatic case. In the bichromatic case the ionization rates possess the more general symmetry property (φ, θ) ↔ (φ + π, 180° - θ). Therefore, for φ = π we predict the emission of the high-energy electrons in the forward direction (θ = 0°). In a bichromatic field the sidelobe structures are strongly influenced by quantum mechanical interference effects. We also explore the θ-dependence of the ionization rates for different relative phases φ, and for those energies which correspond to the classical cutoff law.