The effect of variable electronic transition moment on the EELS intensity distribution within a vibrational progression: use of the R-centroid approximation to analyze results for the CO B-X and O2B′-X transitions

The CO B-X and O₂ B′-X transitions are analyzed with the help of ab initio MRD-CI calculations. In both cases a violation of a well-known rule in electron energy loss spectra (EELS) is observed, namely that the intensity distribution of vibrational bands is normally independent of both the scattering angle and the incident electron energy. In both cases strongly avoided crossings are shown to be responsible for the unusual appearance of the EELS. The analysis is simplified by making use of the R-centroid approximation, which was originally developed to organize and systematize optical absorption and emission data. It is concluded that a breakdown of the intensity distribution invariance in EELS should occur for a wide class of transitions in which avoided crossings lead to rapid variations in the electronic transition moment as a function of internuclear separation.