Two-Phonon Data

Abstract

In our earlier discussion of the interpretation of the infrared- absorption spectra of perfect alkali halides, we made use of the simplest possible classical treatment. The principal discrepancy between the absorption spectra calculated in this way and those observed experimentally lies in the existence of subsidiary structure in the observed spectra. In this section we wish to describe briefly how this can be explained. Since extensive discussions of the problem have been given by a number of authors,^{(49,50,148–150)} we shall give a semiquantitative discussion of the absorption mechanism and show that the interpretation of the observed side-band structure can be reduced to the computation of certain functions of the eigen-frequencies and eigenvectors of the lattice.