Two-colour three-photon ionization of hydrogen

Based on analytical equations valid in perturbation theory, we study the process of ionization of the ground state of the hydrogen atom by the absorption of three photons, two of them being of the same frequency. Results are given for the total ionization rate and for the angular distribution of the ejected electrons, in a regime of frequencies in which two-colour two-photon ionization is not possible. The case of orthogonal photon polarizations, investigated in some detail, displays a strong dependence on the electron direction. (author)