Abstract

The multiphonon transition theory as formulated by Kubo and Toyozawa provides a general framework for dealing with arbitrary phonon spectra. However, in dealing with concrete problems, single frequency models are widely used. To overcome the inadequacy of over-simplified models and to make theoretical calculations practically feasible, the concept of multi-frequency models is developed. The theoretical basis for the use of the method of steepest descent with multi-frequency models is discussed. It is shown that the use of the contribution of the principal saddle point alone just serves to smooth out artificial singular structures introduced by the model.

© 1984 Optical Society of America