  We investigated the impact of selected cloud condensates in exoplanetary
atmospheres on the polarization of scattered stellar radiation. We considered a
selection of 25 cloud condensates that are expected to be present in extrasolar
planetary atmospheres. Using the three-dimensional Monte Carlo radiative
transfer code POLARIS and assuming Mie scattering theory, we calculated and
studied the net polarization of scattered radiation as a function of planetary
phase angle at optical to near-infrared wavelengths. In addition to the
well-known characteristics in the state of polarization, such as the rainbow
determined by the real part of the refractive index, the behavior of the
underlying imaginary part of the refractive index causes an increase or
decrease in the degree of polarization and a change of sign in the polarization
at a characteristic wavelength. In contrast to Al$_2$O$_3$ and MgFeSiO$_4$,
clouds composed of SiO, MnS, Na$_2$S, or ZnS produce a rapidly decreasing
degree of polarization with increasing wavelength in the context of an
exoplanetary atmosphere. Furthermore, the sign of the polarization changes at a
wavelength of about 0.5 $\mu$m to 0.6 $\mu$m, depending on the specific cloud
condensate. The resulting net polarization is mainly positive for cloud
compositions with large imaginary parts of the refractive index, such as Fe,
FeS, and FeO. In addition, for Fe and FeS clouds, the maximum degree of
polarization at long wavelengths is shifted to larger phase angles than for
FeO. We found that most of these cloud condensates are distinguishable from
each other due to their unique wavelength-dependent complex refractive index.
In particular, an increase or decrease of the net polarization as a function of
wavelength and a change of sign in the polarization at specific wavelengths are
important features for characterizing cloud compositions in exoplanetary
atmospheres.
