  According to the Schiff theorem, the atomic electrons completely screen the
atomic nucleus from an external static electric field. However, this is not the
case if the field is time-dependent. Electronic orbitals in atoms either shield
the nucleus from an oscillating electric field when the frequency of the field
is off the atomic resonances or enhance this field when its frequency
approaches an atomic transition energy. In molecules, not only electronic, but
also rotational and vibrational states are responsible for the screening of
oscillating electric fields. As will be shown in this paper, the screening of a
low-frequency field inside molecules is much weaker than it appears in atoms
owing to the molecular ro-vibrational states. We systematically study the
screening of oscillating electric fields inside diatomic molecules in different
frequency regimes,i.e., when the field's frequency is either of order of
ro-vibrational or electronic transition frequencies. In the resonance case, we
demonstrate that the microwave-frequency electric field may be enhanced up to
six orders in magnitude due to ro-vibrational states. We also derive the
general formulae for the screening and resonance enhancement of oscillating
electric field in polyatomic molecules. Possible applications of these results
include nuclear electric dipole moment measurements and stimulation of nuclear
reactions by laser light.
