The infra-red absorption spectrum of silicon carbide
The absorption spectra of crystals of silicon carbide, studied in the range of 1μ to 20μ, reveal besides an intense region of absorption from 10μ to 14μ in which the feflexion maximum at 12μ falls, several comparatively well-defined absorption maxima at shorter wavelengths,viz., at 3·5μ, 5·0μ, 6·3μ to 6·5μ, 7·2μ and 7·7μ in a typical case.
Silicon carbide exists in various crystal forms. From a group theoretical analysis applied to a few typical cases it emerges that the normal modes of the uniaxial silicon carbide consist of movements of silicons and carbons along the optic axis or normal to it.
The reflexion maximum at 12μ, and the Raman line at 818 cm.−1 (12·2μ) can be attributed to an oscillation in which carbons and silicons vibrate in opposite phases. The existence of numerous other vibrations besides this one accounts for the extension of the absorption band beyond 12μ. The absorption maxima in the short wavelengths can be attributed to first order and second order combinations and overtones of these fundamentals.