Abstract
This chapter illustrates the separation of electronic and nuclear motions. The gas phase absorption spectrum of the hydrogen atom and of the hydrogen molecule has been recorded in the ultraviolet, visible, infrared and microwave range. The spectrum of the hydrogen atom consists of separated narrow absorption lines. Nuclei are thousands times heavier than the electrons. In a polyatomic system, while the nucleus moves a little, an electron can travel many times through the molecule. It can be assumed that the electronic motion in a field can be created by immobile nuclei. This concept is called adiabatic approximation, in which the motions of the electrons and the nuclei are separated. The separation of the electronic and nuclear motions represents a fundamental approximation of quantum chemistry. The adiabatic approximation offers a simple 3D model of a molecule which is an extremely useful concept with great interpretative potential.
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