Infra-Red Spectrum of Diamond

THE new theory of crystal dynamics put forward by Sir C. V. Raman1 leads in the case of diamond to the result that the atomic vibration spectrum of this crystal should exhibit eight distinct monochromatic frequencies. Of these, the mode of highest frequency $(1,332 cm^{-1})$ corresponds to the triply degenerate vibration of the two Bravais lattices of carbon atoms with respect to each other, and this is active or inactive in infra-red absorption according as the crystal structure of the diamond possesses tetrahedral or octahedral symmetry. The other seven frequencies represent oscillations of the layers of carbon atoms parallel to the faces of the octahedron or the cube occurring normal or tangential to these planes, with the phase reversed at each successive equivalent layer. By the nature of the case, such oscillations should be inactive in the infra-red absorption as fundamentals; but owing to anharmonicity, the octaves of these frequencies should be capable of appearing in infra-red absorption. Besides the octaves, various combinations of these frequencies would also be active in Infra-red absorption