Knowledge Bank

The principal components of the polarizability for moat common gases and vapors have been determined by Kerr effect measurements. However, experimental difficulties have precluded successful results in the case of water vapor. For $C2v$ asymmetric top molecules, there are two irreducible tensor components of the polarizability tensor which transform according to the totally symmetric representation. Both of these components ``connect” the same rotational states causing the rotational Raman line intensities to be dependent upon their relative sign and magnitude. Therefore, the contour of the rotational Raman spectrum, the depolarization ratio of the Rayleigh scattering, and the refractive index provide three independent experimental results which uniquely determine the three principal polarizability components. Using a computer program written to calculate Raman intensity contours for asymmetric tops, the calculated contour for water is found to be quite sensitive to the relative values of the two irreducible tensor components, and the observed spectrum has been accurately reproduced. The optimum tensor component ratio is combined with depolarization ratio results and the values of the mean polarizability known from refractive index measurements to derive the principal polarizability tensor components.