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 and , and the relative phase 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 between the momentum of the ionized electron and the polarization vector of the laser field. We found that the plateau for and for the backward emission of electrons extends up to , where is the ponderomotive energy of the first laser field component (assuming equal intensities of both components). There are no such high-energy electrons for , in contrast to the symmetry , valid in the monochromatic case. In the bichromatic case the ionization rates possess the more general symmetry property . Therefore, for we predict the emission of the high-energy electrons in the forward direction . In a bichromatic field the sidelobe structures are strongly influenced by quantum mechanical interference effects. We also explore the -dependence of the ionization rates for different relative phases , and for those energies which correspond to the classical cutoff law.