Strong-field multiphoton ionization of hydrogen: Nondipolar asymmetry and ponderomotive scattering

We present an extension of the strong-field theory of multiphoton ionization due to Reiss aimed at retaining the retardation effects. The time-reversed S-matrix approach and a recently proposed strong-field solution to the Schrodinger equation are used in order to obtain analytic expressions for the ionization fates without making the dipole approximation. Photoionization of the hydrogen ground state is used as an example to demonstrate that deviation from the dipole approximation is substantial for high photon orders or, equivalently, highly energetic photoelectrons. As a by-product, an expression is also obtained for the ponderomotive scattering angle of the photoelectron, measured relative to the field propagation direction, in terms of its directed kinetic and binding energies. [S1050-2947(97)07111-4].