The method of successive approximations is used to obtain a solution f
or an infinite chain of nonlinear coupled equations for the Fourier tr
ansform of the retarded two-time Green's function in a system in which
the electron vibrational interactions of the lattice photons and of t
he intramolecular photons are included in the terms of the Hamiltonian
that are diagonal in the electron operators. A unique subsequence, wh
ich provides a finite contribution to the integral intensity of the ab
sorption spectrum, is extracted from the infinite series and summed. T
he resulting formulas are used to classify the excited states of a cry
stal and interpret the cases of anomalous light absorption, when the a
bsorption intensity becomes inversely proportional to the Franck-Condo
n overlap integral. (C) 1997 American Institute of Physics. [S1063-776
1(97)01709-5].