Anisotropy parameters in two-colour two-photon above threshold ionization

We employ the time-dependent R-Matrix (TDRM) method to calculate anisotropy parameters for positive and negative sidebands of selected harmonics generated by two-color two-photon above threshold ionization of argon. We consider odd harmonics of an 800 nm field ranging from the 13th to 19th harmonic, overlapped by a fundamental 800 nm IR field. The anisotropy parameters obtained using the TDRM method are compared with those obtained using a second order perturbation theory with a model potential (MP) approach and a soft photon approximation (SPA) approach. Where available, a comparison is also made to published experimental results. All three theoretical approaches provide similar values for anisotropy parameters. The TDRM approach obtains values that are closest to published experimental values. At high photon energies, the differences between each of the theoretical methods become less significant.