Kinematic Frenkel biexcitons

We consider the two-particle spectra of Frenkel excitons in molecular crystals and demonstrate that, even if the dynamical interaction is weak or absent, bound states of two excitons may be formed due to the kinematic interaction induced by their paulion nature. A simple model of a linear molecular chain with two molecules in the unit cell is studied, which may be applicable to J-aggregates. The number of exciton bands turns our to be crucial for the possibility of kinematic biexciton formation. We also estimate the biexciton contribution to the resonant part of the nonlinear optical polarizability and two-photon absorption, and analyze the stability of the kinematic biexcitons with respect to the dynamical exciton-exciton interaction. (C) 2000 Elsevier Science B,V. All rights reserved