A. ANALYSIS OF REFLECTION SPECTRA BY MEANS OF A FIT WITH A CLASSICAL DISPERSION FORMULA

In the infrared region polar crystals exhibit a reflection spectrum with reststrahlen bands because of the infrared active lattice modes. These modes, which have a linear dipole moment with respect to the normal coordinate, cause the infrared dispersion. When the reflectivity of such a crystal has been measured over a sufficiently wide fre-quency range, two kinds of analyses are generally used to extract the available infor-mation, such as eigenfrequencies, oscillator strengths, and so forth. One method is the Kramers-Kronig analysis,I and the other is an optimum fit of the data by rneans of a classical dispersion formula. The latter method will be discussed in this report with respect to some theoretical aspects and its practical application. Polar and cubic crystals with two particles per unit cell have one infrared active lattice mode. The classical treatment of the interaction of electromagnetic waves and this lattice mode yields the classical dispersion formula 2 for the complex dielectric con-stant of the crystal as a function of the frequency w E = E '- iE&quot