EXCITON SELF-TRAPPING PROCESSES

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 modes of the inter-ionic/molecular vibrations. It is considered here that the symmetric excitons arise from the coupling with symmetry-retaining vibrational modes and vibronic excitons from the coupling to symmetry-breaking modes. Three examples are discussed: (i) formation of the barriers of exciton self trapping, as it occurs in the dimer formation at semiconductor surfaces, (ii) parity breaking that occurs at a centrosymmetric site and leads to an inversion electric dipole which enhances the polarizability and binding energy for pairing vibronic excitons, and (iii) a Bose condensation model of quasi-2D excitons applicable to the layered materials.