Non-dipole effects in the angular distribution of photoelectrons in sequential two-photon atomic double ionization

Motivated by the recent achievements of experiments using x-ray free electron lasers, we have developed a theory for the angular distributions of photoelectrons in sequential two-photon double ionization (2PDI) of atoms beyond the dipole approximation. Expressions for the angular distributions are obtained taking into account the full multipole expansion of radiation in electric and magnetic moments. The formalism is further specified for the first-order corrections to the dipole approximation. As an illustrative example, the sequential 2PDI of the 2p shell in atomic neon is studied. The numerical calculations predict distinct non-dipole effects observable in experiments at present x-ray free electron lasers.