EFFECT OF ISOTOPIC COMPOSITION ON INFRARED ABSORPTION OF THIN FILMS OF LITHIUM FLUORIDE AND LITHIUM HYDRIDE
The interaction of electromagnetic radiation with crystalline lattices is studied by observing the absorption of far-infrared radiatron in thin films of certain simple substances evaporated on various substrates. To elucidate the aature of lattice vibrations, the isotopic composition of the absorbing substance is varied. The substances studied are lithium fiuoride made from varying proportions of Li/sub 6/ and Li/sup 7/ in combination with Fr/sup 9/, and lithium hydride made from varying proportions of Li/sub 6/ and Li/sup 7/ in combination with H/sup 1/ and H/sup 2/. The substrates are polyethylene, potassium bromide, and cesium bromide. Observations are made in the wavelength region l0 to 40 microns, and at film temperatures of 300, 200, and 120 deg K. Each resulting spectrum is inspected for a principal absorption line and for subsidiary - lines. The shape of the principal line is examined, and the variation of the parameters characterizing it, are studied with respect to isotopic composition, temperature, and nature of the substrate. So far as dependence on isotopic mass is concerned, the position of maximum absorption for isotopically pure LiF and LiH follows quantitatively the predictions of the elementary portion of the Born theory, namely, that the dispersion wavelength is proportional to the square root of the reduced mass. The positions of the absorption maxima for rsotopically impure LiF/sup 19/ are intermediate between those for pure Li/sup 6/F/sup 19/ and Lis/ sup 7/F/sup 19/, an d are in agreement with the elementary Born theory if the average isotopic masses are used. The positions of the absorption maxima for isotopically impure LiH do not follow any simple law. So far as dependence on temperature is concerned, the dispersion wavelengths for pure and impure LiF and LiH (with the exception of Li/sup 6/H/sup 1/r and Li/sup 7/H/sup 1/) decr ease slightly with decreasing temperature, as would be expected theoretically. The absorption bands of all the pure and impure LiF and LiH become deeper and narrower with decreasing temperature, in qualitative agreement with theory. The effect of substrate, though not pronounced, is complicated and difficult to understand. It may be concluded generally that although the main features of the interaction of electromagnetic radiation with crystalline lattices are understood, most of the details cannot be predicted quantitatively, and great improvement in the theory is needed.
- Research Organization:
- Michigan State Univ., East Lansing
- NSA Number:
- NSA-16-023690
- OSTI ID:
- 4819243
- Journal Information:
- Dissertation Abstr., Vol. Vol: 22; Other Information: Orig. Receipt Date: 31-DEC-62
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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ABSORPTION
BORN-MAYER EQUATION
CESIUM BROMIDES
COMPOUNDS
CRYSTALS
DEUTERIUM COMPOUNDS
DIFFRACTION
ELECTROMAGNETIC WAVES
EVAPORATION
FILMS
FLUORINE 19
FREQUENCY
HYDROGEN COMPOUNDS
IMPURITIES
INFRARED RADIATION
INTERACTIONS
ISOTOPE EFFECTS
ISOTOPES
LATTICES
LITHIUM 6
LITHIUM 7
LITHIUM FLUORIDES
LITHIUM HYDRIDES
MASS
OSCILLATIONS
POLYETHYLENES
POTASSIUM BROMIDES
RADIATIONS
SPECTRA
SPECTROSCOPY
SURFACES
TEMPERATURE