The vibrations of the MgO crystal structure and its infra-red absorption spectrum
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Records have been made of the infra-red absorption spectra between 5µ and 24µ of magnesium oxide in the form of polished cleavage plates of various thicknesses ranging from 7·5 millimetres down to 20 microns. Films with an effective absorption path ranging from 1 micron to 16 microns have also been prepared by two different techniques and studied. They give results in close agreement with each other.
A critical comparative study of the data reveals that the infra-red absorption by MgO has its origin in a set of distinct modes of vibration of the crystal structure possessing discrete frequencies. The highest of these frequencies corresponds to an infra-red wavelength 20·40µ. It is strongly active as a fundamental and also as overtones. The infra-red absorption spectra enable us to recognize the presence of three other modes with lower frequencies which are inactive as fundamentals but are strongly active as overtones. The characteristic frequencies of MgO (including the highest) revealed by the investigation are respectively in wave-numbers 490 cm.−1, 423 cm.−1, 365 cm.−1 and 313 cm.−1 The corresponding wavelengths are 20·40µ, 23·64µ, 27·4µ and 32µ.
The present investigation shows that the results reported earlier by other authors which were obtained with fumed MgO films are spurious and hence that all theoretical discussions based thereon are valueless.
KeywordsSpectral Region Rabie Transmission Curve Absorption Path Cleavage Plate
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