Kinematic quasi-vacancy Frenkel biexciton in a Davydov multiplet

This paper shows that, as a consequence of paulion-type Frenkel excitons in molecular crystals such as anthracene, with several molecules per unit cell, bound biexcitons exist in the absence of dynamic exciton-exciton attraction. The coupling of excitons in ideal crystals of this type is brought about by the same kinematic effect that causes excitons in nonideal crystals to be localized close to defects such as vacancies. In the excitation spectrum, the resonance peak of the resulting biexciton is found inside a band corresponding to the sum of the energies of excitons of unlike components of a Davydov doublet, located between the bands of the total energies of excitons of the like components of the doublet. The corresponding dispersion equation is obtained, and the energy and wave function of the biexciton is determined.