Multiphoton ionization of a hydrogen atom

The multiphoton ionization of a hydrogen atom is investigated in terms of electron-photon interactions on the basis of perturbation theory. A Sturm expansion of the Coulomb Green's function is used in calculations of the radial amplitudes of multiphoton processes. The corresponding matrix elements are written in compact form by means of hypergeometric functions. Results are given for numerical calculations of the cross sections for two- and three-photon ionization from the ground and excited ns stat5t of a hydrogen atom.