The infra-red absorption spectrum of diamond

Abstract

The absorption spectra of four typical diamonds, three of them luminescent and the fourth non-luminescent, have been investigated in the infra-red in the wavelength range 4·5 μ to 13 μ with a rock-salt prism spectrometer and radiomicrometer. Allowing for reflection, the non-luminescent diamond is found to be completely non-absorbing in the frequency range 770 cm.−1 to 1400 cm.−1. The luminescent diamonds show a band in this region with a number of absorption maxima whose frequencies may be identitied with those of the lattice and superlattice vibrations. The intensity of this band is greatest in the weakly blue-fluorescent diamond, somewhat weaker in the intensely blue-fluorescent diamond and still weaker in the diamond showing both blue and yellow fluorescence. These results are accounted for on the basis of the crystal structure and symmetry of diamond in its various allotropic modifications. The absorption peaks appearing in the region between 1400 to 2700 cm.−1 are accounted for as octaves and combinations of the lattice and superlattice frequencies which are allowed by virtue of the anharmonicity of the vibrations. The appearance of the superlattice frequencies in the 8 μ band is closely associated with the infra-red activity of the fundamental lattice vibration and is in the nature of a resonance effect.