Relativistic effect of atomic radiative processes

We present a theoretical study of the relativistic effect of atomic radiative processes. In order to avoid any entanglement of the atomic screening effect, we would like to elucidate the relativistic effect of the atomic radiative processes by examining the radiative processes of the hydrogenlike atomic ion system. We discover a type of ``zero'' at which the electric dipole matrix element vanishes for n\ensuremath{\kappa}\ensuremath{\rightarrow}\ensuremath{\epsilon}\ensuremath{\kappa}' transitions with \ensuremath{\Vert}\ensuremath{\kappa}\ensuremath{\Vert}=\ensuremath{\Vert}\ensuremath{\kappa}'\ensuremath{\Vert} owing to the relativistic retardation effect. Such a ``zero'' serves as a precursor of a transition of the photoionization cross section from nonrelativistic behavior to relativistic behavior.