Quasidegenerate self-trapping in one-dimensional charge transfer exciton

The self-trapping by the nondiagonal particle-phonon interaction between tw o quasidegenerate energy levels of the excitonic system is studied. We prop ose this is realized in the charge-transfer exciton, where the directions o f the polarization give the quasidegeneracy. It is shown that this mechanis m, unlike the conventional diagonal one, allows a coexistence and resonance of the free and self-trapped states even in one-dimensional systems and a quantitative theory for the optical properties (light absorption and time-r esolved luminescence) of the resonating states is presented. This theory gi ves a consistent resolution for the long-standing puzzles in quasi-one-dime nsional compound A-PMDA.