  When an atom is placed into an oscillating electric field with frequency far
from atomic resonances, the atomic electrons partly shield this field at the
nucleus. It is conjectured that when the frequency of electric field reaches an
atomic resonance, the electric field at the nucleus may be significantly
enhanced. In this paper, we systematically study the mechanisms of this
enhancement and show that it may reach five orders in magnitude in particular
cases. As an application, we consider laser-assisted neutron capture in
139-Lanthanum nucleus and screening and resonance enhancement of nuclear
electromagnetic transitions by electrons.
