A method has been developed of deriving the fundamental absorption spectra of solids directly from experimental infrared data without having to resort to computational evaluation. These characteristic lattice absorption spectra represent the true energy absorption spectra of solids. As opposed to the traditional absorption coefficient (k) vs frequency curve, which is regarded as the absorption spectrum for solids in the region of anomalous dispersion, the spectra, as derived by the proposed method, clearly resolve the transverse and longitudinal modes of vibration. They also exhibit the characteristics of anharmonicity and damping. The method uses the combined reflection and transmission data on single-crystal and thin-film specimens. Its validity has been verified on a wide variety of solids. The most recent data obtained from emission, laser-Raman, or cold neutron scattering techniques, is in complete agreement with the derived values. A method of calculating the various modes of vibration of solids, directly from elastic constants, is also advanced. It is simultaneously used to render further support for the developed characteristic energy absorption spectra. The importance of these spectra is discussed with regard to some problems in solid state physics.
J. N. Plendl, "Characteristic Spectra of Energy Absorption for Dielectric Solids," Appl. Opt. 9, 2768-2786 (1970)