INFRARED SPECTRA OF CRYSTALS.
Physical sciences research papers,
AIR FORCE CAMBRIDGE RESEARCH LABS L G HANSCOM FIELD MASS
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The infrared absorption and reflection in a crystal due to the interaction of the incident electromagnetic field with its vibrational modes are described in this article. The concept of normal vibrations of a crystal is introduced by the example of a linear chain, and then is generalized to more complex 3-dimensional crystals. Methods of classifying unit cell modes by means of factor and site group analyses are pointed out. Dispersion of infrared radiation by an ionic crystal producing the reststrahlen spectrum is treated next, followed by the method of determination of optical constants from such a spectrum and also from attenuated total reflection and emission spectra. The role of vector k not equal 0 modes, where k 2 pilambda represents the wave vector, in the interaction of more than one vibrational mode with the incident photon resulting in combination and overtone bands is discussed. Special emphasis is given on the optically active multiphonon processes in simple crystals. Examples are furnished by a discussion of the lattice infrared spectrum of AlSb followed by a comprehensive review of the multiphonon infrared absorption in diatomic cubic crystals. Infrared spectra of gypsum, CaSO4 2H2O and solid cyclopropane single crystals are described next as examples of more complex crystals. Author