THE INFRARED SPECTRUM OF RHOMBIC SULFUR IN THE ROCK SALT REGION: A STUDY OF THE EFFECT OF TEMPERATURE ON THE INTENSITY OF OVERTONE AND COMBINATION $BANDS^{*}$
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Date
1961
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Ohio State University
Abstract
Integrated relative intensities have been measured as a function of temperature and sample orientation for some overtone and combination bands in single crystals of rhombic Sulfur. Intensities were measured for bands at $845 cm^{-1}$, $903 cm^{-1}$, $936 cm^{-1}$, and $1513 cm^{-1}$ over a temperature range from $85^\circ$ C, to -- $185^\circ$ C. An expression is derived which determines the dependence of the integrated intensity on temperature for overtone and combination bands in molecular crystals. The assumptions are made that the molecules in the crystal may be treated as essentially independent units and that the higher harmonics arise from nonlinear terms in the dipole moment expansion. The component frequencies of the above overtone and combination hands have been assigned on the basis of a comparison of experimental and predicted values of integrated intensity as a function of temperature. The results of these measurements are used to establish rough values for some of the higher fundamental frequencies. In particular, it is found that two of the previously assigned fundamentals, $532 cm^{-1}$ ($B_{1}$) and $520 cm^{-1}$ ($E_{3}$), should be changed to approximately $411 cm^{-1}$ ($B_{1}$) and $434 cm^{-1}$ ($E_{3}$) respectively. In addition, the structure of the bread band at $845 cm^{-1}$ has been studied from the point of view that band structure in molecular crystals is due to combination with acoustic lattice modes The temperature dependence at points well out on the tail of the band agrees with that predicted from a Boltzman distribution of equally spaced energy states, while that in the region close to the central frequency does not This may be explained by the fact that, in the case of lattice combinations, mechanical anharmonicity plays the predominant role in determining band structure.
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$^{*}$This work was supported by a grant from National Science Foundation. $^{**}$ Present address: The General Tire \& Rubber Company, Akron, Ohio.
Author Institution: Chemistry, Department, Syracuse University
Author Institution: Chemistry, Department, Syracuse University