Symmetry states of vibronic Frenkel excitons

Small-radius Frenkel excitons coupled with phonons can have diverse configurational structures depending on the symmetry of the electronic states involved and that of the associated inter-ionic vibrations. A theory is presented for the excitonic polarons arising from the coupling with the symmetry retaining vibrational modes and vibronic excitons arising from the coupling with symmetry-breaking modes. Simple models for each type are presented and their results are related with those of specific experimental situations. Implications for the defect-formation processes under ionizing conditions are discussed.