  The quantum interference between the fine structure states of an atom
modifies the shapes of the emergent Stokes profiles in the Second Solar
Spectrum. This phenomenon has been studied in great detail both in the presence
and absence of magnetic fields. By assuming a flat-spectrum for the incident
radiation, the signatures of this effect have been explored for arbitrary field
strengths. Even though the theory which takes into account the frequency
dependence of the incident radiation is well developed, it is restricted to the
regime in which the magnetic splitting is much smaller than the fine structure
splitting. In the present paper, we carry out a generalization of our
scattering matrix formalism including the effects of partial frequency
redistribution (PRD) for arbitrary magnetic fields. We test the formalism using
available benchmarks for special cases. In particular we apply it to the
Li\,{\sc i} 6708\,\AA\ D$_1$ and D$_2$ line system, for which observable
effects from the Paschen-Back regime are expected in the Sun's spectrum.

Solar and Stellar Astrophysics