ON THE DETERMINATION OF VIBRATIONAL RAMAN SCATTERING CROSS SECTIONS AND POLARIZABILITY MATRIX ELEMENTS: PARTIALLY FLUORINATED METHANES
Creators:Murphy, W. F.
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Publisher:Ohio State University
The measurement of the spectral data needed to determine scattering cross sections for vibrational Raman transitions in gas phase molecules can usually be accomplished without great difficulty using currently available techniques. However, it is often necessary to separate contributions from two or more vibrational modes whose bands overlap, or to separate the contributions to a single mode by two or more polarizability matrix elements. This problem occurs in the evaluation of the polarizability matrix elements of the partially fluorinated methanes: methyl fluoride, methylene fluoride and fluoroform, and various approaches may be taken to solve it. Intensity data for several isotopically substituted species can be analyzed simultaneously to find such contributions. Relative values of polarizability matrix elements can be obtained in an analysis of high resolution vibration-rotation spectra of overlapping or degenerate bands. Recent results from ab initio Raman intensity calculations reproduce observed values with acceptable accuracy; such results, if available, may provide guidance in separating contributions from different polarizability matrix elements. In the present instance, an attempt is being made to estimate the contributions of the different polarizability matrix elements to the scattering cross sections of the partially fluorinated methanes using a bond polarizability model. Initial attempts to exploit the transferability of bond parameters between these related molecules have been disappointing. The current status of this investigation will be presented.
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