The influence of high power resonance electromagnetic radiation on a F
renkel exciton system is studied. By means of the Keldysh diagram tech
nique a formula for the nonlinear susceptibility is obtained. The infl
uence of the excitation intensity on the shape of exciton band and on
the value of the absorption coefficient is established. The phenomena
of optical bistability and multistability are considered taking into a
ccount the nonlinear boundary conditions.