Kinematic Frenkel biexcitons

We consider the two-particle spectra of Frenkel excitons in molecular cryst als and demonstrate that, even if the dynamical interaction is weak or abse nt, bound states of two excitons may be formed due to the kinematic interac tion 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 applicab le to J-aggregates. The number of exciton bands turns our to be crucial for the possibility of kinematic biexciton formation. We also estimate the bie xciton contribution to the resonant part of the nonlinear optical polarizab ility 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.