Photoelectron angular distribution in bichromatic atomic ionization

Theoretical aspects of photoelectron angular distributions are discussed with reference to bichromatic ionization by an FEL beam. Specifically, we consider an asymmetry in the angular distribution due to interference between the ionization paths from the fundamental and the second harmonic. The case of circularly polarized radiation is analyzed in detail in the vicinity of an intermediate resonance in the two-photon ionization paths. Similarities and differences to interference phenomena due to non-dipole effects are also discussed. (paper)