Abstract
We formulate the problem of the calculation of depolarizing isotropic collisions, which are a fundamental ingredient for realistic studies of solar polarization based on the Hanle effect. We present a semi-classical collisional theory that we developed over the last few years, and applied to all simple atoms and to some specific cases of complex atoms. A fundamental result that justifies our approach is that the depolarization process depends strongly on the intermediate range of the interatomic separations, which are accurately described by our semi-classical approach. We present the first results obtained for the collisional depolarization and polarization of rovibrational levels of diatomic molecules. We computed depolarization and polarization transfer rates for all rotational levels J = 1,..., 99 with vibrational quantum numbers v = 0; 1; 2, 3 of the SiO molecule. This could help understanding the role of isotropic collisions in the formation of the very interesting polarized lines of CZ and MgH observed in the solar photosphere. Current spectro-polarimetric observations allow us to learn a lot about the physical conditions of the medium where the spectral lines form, provided that we understand the underlying physics. In this context, we draw the attention to the importance of collisions in solar physics, and in astrophysics in general, in order to keep abreast of the observational advances brought by new instrumentation.