  Quantum interference effects play a vital role in shaping the linear
polarization profiles of solar spectral lines. The Ba II D2 line at 4554 A is a
prominent example, where the F-state interference effects due to the odd
isotopes produce polarization profiles, which are very different from those of
the even isotopes that have no F-state interference. It is therefore necessary
to account for the contributions from the different isotopes to understand the
observed linear polarization profiles of this line. Here we do radiative
transfer modeling with partial frequency redistribution (PRD) of such
observations while accounting for the interference effects and isotope
composition. The Ba II D2 polarization profile is found to be strongly governed
by the PRD mechanism. We show how a full PRD treatment succeeds in reproducing
the observations, while complete frequency redistribution (CRD) alone fails to
produce polarization profiles that have any resemblance with the observed ones.
However, we also find that the line center polarization is sensitive to the
temperature structure of the model atmosphere. To obtain a good fit to the line
center peak of the observed Stokes Q/I profile, a small modification of the
FALX model atmosphere is needed, by lowering the temperature in the
line-forming layers. Because of the pronounced temperature sensitivity of the
Ba II D2 line it may not be a suitable tool for Hanle magnetic-field
diagnostics of the solar chromosphere, because there is currently no
straightforward way to separate the temperature and magnetic-field effects from
each other.
