Raman Intensities and the Nature of the Chemical Bond

Abstract

The experimental determination of Raman intensities is commonly a difficult procedure, particularly when the resulting intensities are to be placed on an absolute scale. For the purpose of evaluating derived properties characteristic of the chemical bonds responsible for the intensities, Raman intensities which have been corrected for such factors as spectrometer spectral sensitivity, optical absorption, sample geometry, and refractive index must be used. However, before discussing these factors and the applications and interpretations of absolute Raman intensities, it is necessary to examine in some detail the various theoretical relationships used to relate such corrected intensities to properties of chemical bonds. THEORY OF RAMAN INTENSITIES In the introductory chapter of this volume, it was shown that a quantum mechanical expression 1 for the intensity of Raman scattering by (nonrotating) molecules may be written as I = Nn63:~2 (vo-~v)4[P];m (1) where N n is the number of molecules in an initial state characterized by a set of vibrational quantum numbers n, Vo is the frequency of the incident light, ~ v is frequency shift on scattering, and P nm is the transition probability linking the initial states of the molecules to the final states, characterized by quantum numbers m. The transition probability factor is expressed in terms of the induced dipole moment vector P, which in turn is related to the molecular polarizability IX