Anharmonic effects in the dispersion of the permittivity tensor of solid solutions

A study is made of the influence of weak anharmonieity on the dispersion of the complex permittivity tensor of solid solutions in the neighborhood of light absorption by impurity centers with allowance for the change of the quasielastic constants when phototransitions take place. To calculate the resulting correlation functions, which contain four boson operators, a method is proposed on the basis of a Laplace transformation; this appreciably simplifies the calculations compared with the method of Green's functions.