Line Mixing in the Rotational Raman Spectrum of Ethylene.

The principles of line mixing are applied in an approximate way to the rotational Raman spectrum of the nearly symmetric top, ethylene. The results of this application are compared to experimental data. The line mixing model used was developed from the qualitative properties of the theory of pressure broadening of overlapping lines. Past experimentation has shown that spectral lines which overlap significantly form a band which narrows as pressure is increased. Following this property an approximate model of line mixing is developed. The model consists of assigning Lorentz lineshapes to all lines that overlap significantly with line-widths that vary inversely with pressure. The lines which are affected most by this model are the symmetric top allowed transitions. These lines are degenerate for the symmetric top and slightly separated for the nearly symmetry top. The free spectral range of a Fabry-Perot interferometer is set to match the separation of the rotational lines of the symmetric top. This allows the lines of the rotational Raman spectrum to pass through the interferometer simultaneously. The shape of the interferogram is sensitive to the thermodynamic properties of the gas. It is shown to be particularly sensitive to the effects of line mixing. Experimental interferograms were taken for a range of pressures less than 1.5 atmospheres and temperatures from 25(DEGREES)C to 100(DEGREES)C. It is demonstrated that theoretical interferograms calculated with the proposed line mixing model agree well with the experimental interferograms.