We study relativistic effects in photoionization of a hydrogen atom exposed to an intense laser pulse of general ellipticity. The frequency of the laser pulse resides in the ultraviolet region. To this end, the semirelativistic approach introduced in Kjellsson Lindblom et al. [T. Kjellsson Lindblom, M. Forre, E. Lindroth, and S. Selsto, Phys. Rev. Lett. 121, 253202 (2018)] is applied. We present in some detail how this approximation is derived from the Dirac equation for elliptically polarized light within the so-called long-wavelength approximation. The validity of the semirelativistic approach is confirmed by direct comparison with the solution of the Dirac equation. It is found that the total ionization yield depends very weakly on ellipticity in the case of ionization from the isotropic ground state. With the excited initial state n = 2, l = m(l) = 1; however, pronounced ellipticity dependence is seen-in particular at the stabilization peak. Albeit small, relativistic corrections to the ionization probabilities are found. The correction is found to be largest for linear polarization. While relativistic effects tend to reduce the total ionization probability for most intensities considered, we also report a slight relativistic enhancement at comparatively modest field strengths.

We study the photoionization process of a hydrogen atom initially prepared in a circular Rydberg state. The atom is exposed to a two-cycle laser pulse with a central wavelength of 800 nm. Before the atom approaches saturation, at field intensities of the order of 10(17) W/cm(2), relativistic corrections to the ionization probability are clearly seen. The ionization is predominantly driven by the radiation pressure in the propagation direction of the laser field, not by the electric field. Direct comparisons with the full numerical solution of the time-dependent Dirac equation demonstrate quantitative agreement with a semirelativistic approximation, which is considerably easier to implement.