THEORY OF THE IMPACT BROADENING OF RAMAN LINES DUE TO ANISOTROPIC INTERMOLECULAR FORCES

Abstract

The impact theory of Raman line broadening developed previously is applied to the broadening due to anisotropic intermolecular forces. The concept of frequency degeneracy is introduced, and its importance for the broadening of isotropic Raman lines is discussed. The components of a pure vibrational Raman band show a broadening due to the anisotropic intermolecular forces proportional to the density as long as the separation between the components is large compared with the line width. However, if the components of the vibrational band are not resolved the anisotropic forces give no further broadening with increasing density. Explicit expressions are derived for the elastic and inelastic parts of the optical cross section by expanding the collision operator in powers of the interaction responsible for the broadening and retaining only the lowest-order nonvanishing terms, but preserving the unitarity of the collision operator. The dependence of the cross section on the interaction and on the molecular diameter is shown to be quite different for weak and strong interactions.