ABSTRACT
The Born-Oppenheimer approximation is a powerful concept on which much of the material in the next few chapters is based. It provides a physical picture in which nuclear motion takes place on potential energy curves (or surfaces, for polyatomics) belonging to distinct electronic states. The interesting spectroscopic consequences of the breakdown in this approximation will be considered in Chapters 11 and 12. A further approximation to be made is that of the harmonic oscillator (HO) model, which provides potential energy surfaces which are quadratic functions of the vibrational coordinate. The HO model provides a point of departure for considering more realistic potential functions. These more appropriate potentials are said to account for anharmonicity. Using diatomics as the simplest possible examples, some of the interesting consequences of anharmonicity are illustrated. The study of polyatomics reveals additional signatures of these effects.
