Shielding of an external oscillating electric field inside atoms

According to the Schiff theorem, an external electric field vanishes at the atomic nucleus in a neutral atom, i.e., it is completely shielded by electrons. This makes a nuclear electric dipole moment (EDM) unobservable. In this paper an extension of the Schiff theorem to an oscillating electric field is considered. Such a field can reach the nucleus and interact with the nuclear EDM. An enhancement effect appears if the field is in resonance with an atomic or molecular transition. The shielding by electrons strongly affects low-energy nuclear electric dipole transition amplitudes in different nuclear reactions, including radiative transitions, radiative nucleon capture, photo- or electro-excitation of nuclei, and laser-induced or laser-enhanced nuclear reactions.