  The spectral line polarization produced by optically pumped atoms contains a
wealth of information on the thermal and magnetic structure of a variety of
astrophysical plasmas, including that of the solar atmosphere. A correct
decoding of such information from the observed Stokes profiles requires a clear
understanding of the effects that radiatively induced quantum interferences (or
coherences) between pairs of magnetic sublevels produce on these observables,
in the absence and in the presence of magnetic fields of arbitrary strength.
Here we present a detailed theoretical investigation on the role of coherences
between pairs of sublevels pertaining to different fine-structure J-levels,
clarifying when they can be neglected for facilitating the modeling of the
linear polarization produced by scattering processes in spectral lines. To this
end, we apply the quantum theory of spectral line polarization and calculate
the linear polarization patterns of the radiation scattered at 90 degrees by a
slab of stellar atmospheric plasma, taking into account and neglecting the
above-mentioned quantum interferences. Particular attention is given to the
2S-2P, 5S-5P, and 3P-3S multiplets. We point out the observational signatures
of this kind of interferences and analyze their sensitivity to the energy
separation between the interfering levels, to the amount of emissivity in the
background continuum radiation, to lower-level polarization, and to the
presence of a magnetic field. Some interesting applications to the following
spectral lines are also presented: CaII H and K, MgII h and k, NaI D1 and D2,
the BaII 4554A and 4934A resonance lines, the CrI triplet at 5207A, the OI
triplet at 7773A, the MgI b-lines, and the H-alpha and Ly-alpha lines of HI.

Solar and Stellar Astrophysics