  Radiation pressure effects in ionization by short linearly-polarized laser
pulses are investigated in the framework of strong-field approximation, in both
nonrelativistic and relativistic formulations. Differences between both
approaches are discussed, and retardation and recoil corrections are defined.
It is demonstrated how these corrections can be incorporated into the
nonrelativistic approach, leading to the so-called quasi-relativistic
formulation. These three approaches are further applied to the analysis of
signatures of radiation pressure in energy-angular distributions of
photoelectrons. It is demonstrated that, for Ti:Sapphire laser pulses of
intensities up to $10^{16}\mathrm{W/cm}^2$, predictions of the
quasi-relativistic formulation agree well with those of the full relativistic
one, and that the recoil corrections contribute predominantly to the radiation
pressure effects.
