  According to the Schiff theorem an external electric field vanishes at 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 field can reach the nucleus and interact with the nuclear EDM. The
enhancement effect appears if the field is in resonance with 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.
