ANGULAR-MOMENTUM COUPLING IN SPECTROSCOPIC RELAXATION CROSS-SECTIONS - CONSEQUENCES FOR LINE COUPLING IN BENDING BANDS

In the classical limit and within the infinite-order sudden approximation (IOSA), the tetradic collisional spectroscopic cross sections are transformed into a dyadic supermatrix. Each element of this matrix is expressed in terms of generalized state-to-state cross sections, each row or column corresponding to a given spectroscopic branch. These generalized cross sections are convenient both for ab initio calculations, including possible energy corrections to IOSA, and for modeling. One of the main advantages of the present approach is to give a clear understanding of the line-coupling mechanism and of its spectral consequence on complex systems, whatever the type of spectroscopy (infrared absorption, isotropic or anisotropic Raman scattering) and whatever the nature of the observed vibrational transition (stretching or bending band). To illustrate this point, the strong-collision model is used to calculate explicitly the generalized,state-to-state cross sections. This model allows one to justify, on a first and simple physical basis, the origin of the ad hoc decoupling ''factor of 2'' previously introduced to fit the infrared Q-branch profiles for bending bands.