  The relativistic Hartree approximation predicts a deep attractive potential
for antinucleon, which largely reduce the threshold energies of the
nucleon-antinucleon (N-Nbar) production. This effect has played an important
role to explain the quenching of the Gamow-Teller (GT) strength because the
quenched strength in the particle-hole excitation is partially taken by the
nucleon-antinucleon production. On the other hand antiproton experiments do not
reveal deep attractive potential for antinucleon. In this paper we study
energy-dependence of the nucleon self-energies in the relativistic Hartree-Fock
(RHF) approximation in off-mass-shell states. Then we demonstrate that the
antinucleon appearing in low energy observables is in the off-mass-shell energy
region and its properties are quite different from that at the on-mass-shell
state. Furthermore we show that the quenched amount of the GT strength is not
taken only by the \NNbar production but also by the meson production through
the imaginary part of the nucleon self-energy in the RHF approximation.
