Raman Intensity in Binary Solutions

Abstract

Relative to the intensity in pure solvent, the integrated intensity of a Raman band in solution decreases not linearly with molar concentration, but more or less rapidly as the refractive index of the solution rises or falls on addition of solute. This behavior is treated here as a consequence of excited-state interactions between the components of the solution. With hybrid intermediate states in the perturbation-theory expression for the scattering coefficient, the reported correlations between Raman intensities in solution and refractive index or ultraviolet absorption spectra can be derived. In a Drude type of approximation, the lowest-lying pure electronically allowed transition frequency is the only parameter necessary to reproduce the correlations and also to account for the similarity in behavior among the several Raman bands of any species. The temperature dependence of the water stretching band and the lack of large direct cation effects, even in solutions containing colored cations, can also be interpreted. Some of the illumination factors that must be considered before intensities observed in optically dissimilar solutions may be compared are discussed, and the possible analytical uses of Raman intensities are briefly mentioned.