ABSTRACT
“In most discussions on molecular wave functions, the validity of the Born–Oppenheimer approximation is assumed. This approximation is most nearly accurate when the frequencies of motion, which can be gauged by energy level spacings, are much larger for the electronic than for the nuclear motions. Ever since Mulliken’s work, rovibronic coupling, or as it is sometimes called, rotation/vibration-electron coupling, has remained a central aspect of the study of molecular Rydberg states. Clearly the spectacular increase of the number of channels with which one has to deal in molecules as compared to atoms stems in part from the lower symmetry of molecular systems. The idea of generalizing quantum defect theory to potential fields other than the Coulomb attraction between two point charges arises naturally in molecular physics where the motion of the nuclei is governed by electronic potentials which exhibit a wide variety of different shapes.
