Infra-red absorption spectra of crystals

Abstract

Spectroscopic investigations on the scattering of light in crystals show very plainly that their vibration spectra have a discrete structure manifesting itself as a set of lines with sharply defined frequency shifts. That a similar discrete structure is also to be seen in the infra-red absorption spectra of crystals when examined under appropriate conditions is shown by a remarkable investigation by Barnes and Brattain. These authors examined the case of magnesium oxide, which is a crystal of the cubic class with a structure similar to that of rock-salt. Plates obtained by cleavage from flawless transparent single crystals of large size were employed, and the absorption spectra of a whole series of such plates from the very thinnest obtainable to the thickest were investigated. The rock-salt spectrometer used in the study covered the wave-length range 1-15.6 μ , and the galvanometer deflexions were amplified 150 times by a photo-electric relay system. The resulting data showed the presence of some forty absorption lines in the wave-length range 6-15.5 μ . In the absorption curves reproduced with their paper, these lines are so sharply defined that their positions can be measured with high precision.