Title: Relativistic effects in the non-resonant two-photon K-shell ionization of neutral atoms,
Abstract: Relativistic effects in the non-resonant two-photon K-shell ionization of
neutral atoms are studied theoretically within the framework of second-order
perturbation theory. The non-relativistic results are compared with the
relativistic calculations in the dipole and no-pair approximations as well as
with the complete relativistic approach. The calculations are performed in both
velocity and length gauges. Our results show a significant decrease of the
total cross section for heavy atoms as compared to the non-relativistic
treatment, which is mainly due to the relativistic wavefunction contraction.
The effects of higher multipoles and negative continuum energy states
counteract the relativistic contraction contribution, but are generally much
weaker. While the effects beyond the dipole approximation are equally important
in both gauges, the inclusion of negative continuum energy states visibly
contributes to the total cross section only in the velocity gauge.