Application of Quantum Theory to Spectroscopy

We learn from quantum mechanics that allowed energy states exist in which a molecule or an atom may spend long or short periods of time. A molecule or atom can exist in intermediate energy states for only a transient time when it is ascending or descending from one level (or state) to another. 

For a polyelectron atom, a multiplicity of absorptions is permissible. The energy required to produce a 4p transition (A i) corresponds to visible radiation 2s- 2p (AF a) requires far-ultraviolet radiation and 1j-9-2j (AEg) requires x-ray radiation. 

For polyatomic molecules, electronic transitions involve molecular orbitals such transitions require energy in the ultraviolet region and are of vital importance in ultraviolet spectroscopy. 

The spectrum is a function of the whole structure of a molecule rather than of specific bonds. Photons of low energy (far infrared, microwave) can produce changes of 

Commercial spectroscopic instruments are readily available some of these are very sophisticated, for u e in exacting research studies that require high precision. Many studies do not require such precision for these, simple, less expensive, models are also available. In some cases, part or all of such instruments may be built by the investigator at a considerable saving. 

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