The Infrared and Raman Line Shapes of Pairs of Interacting Molecules

Abstract

Existing line shape computations of collision-induced infrared and Raman spectra are reviewed. The theory of free-free transitions of binary molecular complexes is sketched. For line shape calculations, two extreme standpoints are of a special interest. One attempts the most realistic modeling of the molecular interaction as a basis for critical comparisons of the fundamental theory with measurements, for predicting unknown spectra from theory, for determining empirical induced dipole and polarizability models, and for providing exact quantum profiles to test line shape models. Examples of such computations have no adjustable parameters and agree closely with measured profiles on an absolute intensity scale if spectra are obtained at low densities where binary collisions prevail. The other extreme aims for the generic features of collision-induced spectra from minimal specific assumptions. Selected line shape functions with three or more adjustable parameters are capable of approximating certain spectral components closely, are inexpensive to compute and can be used for temperature interpolation of computed or measured spectra. Various previously proposed model functions differ strikingly with respect to the goodness of fit attainable.