Relativistic ionization probabilities and photoelectron distributions of hydrogenlike ions in superstrong electromagnetic fields

The ionization probabilities and photoelectron distributions of the hydrogen atom and hydrogenlike ions ${\mathrm{He}}^{+}$ and ${\mathrm{Ne}}^{9+}$ in superstrong electromagnetic fields are obtained by solving the time-dependent Dirac equation with the help of the generalized pseudospectral method in spherical coordinates. A simple transformation of the original radial Dirac Hamiltonian is suggested that removes the spurious states. The ionization probabilities are calculated both within and beyond the dipole approximation for two pulse durations and various peak field strengths scaled with respect to the nuclear charge for each target. Performance of two analytic parameters previously suggested to estimate the nondipole effects is assessed against the numerical data obtained in this study. The photoelectron energy and angular distributions exhibit the above-threshold ionization peaks as well as the low-energy structure. In the superstrong field regime, the low-energy structure builds up with increasing field strength while the above-threshold ionization peaks decrease. The nondipole effects in the photoelectron distributions are also revealed and analyzed.