Group theory gives us a framework for predicting allowed transitions. If the product of the ground and excited state symmetries matches the symmetry of one of the polarizations (x, y or z) of radiation there can be an absorption. Even when no absorption is predicted by the usual Franck-Condon mechanism, group theory provides a framework for search for vibronic modes that permit absorption of light in those cases as well. These issues are considered in the presentation on symmetry selection rules.
