Gap kinematic Frenkel biexciton

A general dispersion equation is derived determining the wave-vector depend ence of the energy of a bound kinematic Frenkel biexciton, previously disco vered by the author, in crystals of arbitrary symmetry. A solution of this equation shows that, in crystals of a certain symmetry, bound gap Frenkel b iexcitons can exist, the isolated terms of which lie in the gap between the components of the Davydov multiplet of unbound two-exciton states. In crys tals with two monomers in the unit cell, these terms lie in the gap between two low-frequency components of the Davydov multiplet and are not in reson ance with the central component, in contrast to what was observed in previo usly investigated crystals of higher symmetry. Besides, the biexcitons disc overed in this work have a finite, rather than zero, dispersion, although t heir bandwidth is extremely small. The bound biexcitons also have some othe r specific features followed from the dispersion relation. (C) 2000 MAIK "N auka/Interperiodica".