Optical orientation of excitons in CdS

The optical orientation by circularly polarized light of free and bound excitons in CdS is reported and discussed. The observed degrees of polarization are generally large and vary significantly with the exciting energy (a "negative" polarization is sometimes obtained). These results, together with the effects of a magnetic field, provide a lot of information about the levels excited, the luminescent levels (lifetimes and disorientation times), and the transfer of polarization from the first to the second. The optical orientation by linearly polarized light is also observed for the first time in solids. It is shown why these unusual effects can be seen on free excitons and not on other levels. The additional information that they provide leads to a better comprehension of the mechanisms of thermalization of the excitons, and of the symmetry properties of $K\ne 0$ exciton states.