  We develop a model of off-mass-shell pairing correlations in nuclear systems,
which is based on the meson-exchange picture of nuclear interactions. The
temporal retardations in the model are generated by the Fock-exchange diagrams.
The kernel of the complex gap equation for baryons is related to the in-medium
spectral function of mesons, which is evaluated nonperturbatively in the random
phase approximation. The model is applied to the low-density neutron matter in
neutron star crusts by separating the interaction into a long-range
one-pion-exchange component and a short-range component parametrized in terms
of Landau Fermi liquid parameters. The resulting Eliashberg-type coupled
nonlinear integral equations are solved by an iterative procedure.We find that
the self-energies extend to off-shell energies of the order of several tens of
MeV. At low energies the damping of the neutron pair correlations due to the
coupling to the pionic modes is small, but becomes increasingly important as
the energy is increased. We discuss an improved quasiclassical approximation
under which the numerical solutions are obtained.
