Spin-flip processes and nondipole effects in above-threshold ionization of hydrogen in ultrastrong laser fields

We consider above-threshold ionization of hydrogen atoms in ultrastrong laser fields. We use a time-dependent Dirac equation as a calculational tool. This framework allows one to include relativistic effects such as nondipole effects, effects of the relativistic kinematics, and electron spin effects. Inclusion of the spin effects allows one to consider the spin-flip process accompanying above-threshold ionization. We present and discuss electron momenta distributions for ionization processes with and without spin flips. Electron momenta distributions for the ionization process without a spin flip show a gradual increase of the role of the nondipole effects with increasing electric field. Electron spectra for the spin-flip ionization exhibit cusplike singularities, and the absence of features (such as the presence of dips) in the case of the ionization without a spin flip. We explain these features by invoking perturbation theory arguments.