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Quantum Mechanical Basis 

 of Molecular Spectra 



I. Introduction 



In the last chapter, some of the types of spectrophotometers used in 

 biological research were described. All of these depend on molecular 

 absorption spectra. It is quite customary to use molecular spectra as a 

 tool with little or no basis for understanding the physical origin of these 

 spectra. By contrast, this chapter is intended to present an elementary, 

 physical picture of the molecular basis for absorption spectrophotom- 

 etry. 



A good deal of space and many words are devoted to the various 

 symbols and to the terminology common in spectrophotometry. Al- 

 though these may seem far from biology, discussions of photobiology 

 and of energy transformations in biology use the symbols and concepts 

 of molecular spectroscopy. Thus, these symbols and ideas are important 

 not only for their direct application toward understanding spectros- 

 copy, but also for describing other biologically significant types of events. 



In order to understand molecular-absorption spectra, it is necessary 

 first to have some ideas about molecular-energy levels. These in turn 



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