Optical Absorption by Alkali Halides: Possible Structure in the Multiphonon Region
Earlier theories of the multiphonon absorption in insulators have either treated anharmonicity as a perturbation rather than exactly, or have ignored the lattice in favor of a single-oscillator model. Here we attempt to incorporate the important features of both models into a single theory applicable to many substances. For the heavier alkali halides, the calculation predicts some structure in the absorption spectrum in the “transparent” region well above the reststrahl line. The nature of this structure is examined in detail by focusing specifically upon results for the potassium halides. The amount of structure is shown to be related to the amount of overlap between the optic and acoustic branches of the phonon dispersion curves. Experimental possibilities for observing the multiphonon bumps are discussed.
KeywordsOptical Absorption Dissociation Energy Morse Potential Alkali Halide Phonon Dispersion Curve
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