Nonrelativistic Dirac equation: An application to photoionization of highly charged hydrogenlike ions

We investigate the role of relativity in photoionization of hydrogenlike ions by a laser pulse. For hydrogen, the wavelengths of the laser resides in the weakly ultraviolet region. For higher nuclear charges, the laser parameters are scaled in a manner which renders the time-dependent Schr"odinger equation in the electric dipole approximation independent of nuclear charge. The ionization potentials of these highly charged ions are strongly modified by relativistic effects. In an earlier work by Ivanova et al. [Phys. Rev. A 98, 063402 (2018)], it is demonstrated how this explains most of the relativistic correction to the ionization probability. Here we investigate to what extent remaining discrepancies can be attributed to relativistic effects stemming from the strong external field. To this end, we solve semirelativistic formulations of both the Schr"odinger and the Dirac equations; the former accounts for increased inertia due to the external laser field, while the latter features a nonrelativistic interaction term.