502 Quantum Mechanical Basis of Molecular Spectra /27 : 2 



can be adequately described only in the language of quantum mechanics. 

 The ideas and theories of quantum mechanics have been supported by a 

 wide variety of experiments, such as those dealing with characteristic 

 spectra, specific heats, and the photo-electric effect. The original 

 observations which most forcefully demanded the creation of quantum 

 mechanics for their explanation were the regularities of the character- 

 istic spectra of atoms. It seems quite fitting to present a discussion of 

 quantum mechanics in a chapter on spectrophotometry. 



2. An Elementary Approach to Quantum 

 Mechanics 



A rigorous presentation of the theory of quantum mechanics is far beyond 

 the scope of this text. Rather, it is hoped that this discussion will serve 

 to acquaint biologically oriented readers with the concepts of quantum 

 mechanics, and to orient the physically inclined to thinking of quantum 

 mechanics in terms of its application to spectra of biological materials. 



One of the fundamental ideas of quantum mechanics is that many 

 physical quantities such as energy, momentum, volume, and mass come 

 in small finite chunks, called quanta. It is not permissible even to think 

 of further subdividing these quanta. They are all so very small that in 

 the macroscopic world one is not generally aware of their existence. 

 However, on a submicroscopic scale, quantum mechanics is the only 

 theory which correctly predicts the behavior of small molecules, atoms, 

 electrons, and subatomic particles. The absorption spectra of interest 

 in biophysics involve changes within molecules. Hence, one can hope 

 to understand these spectra only in terms of quantum mechanics. 



The idea that mass also comes in chunks is familiar to all readers. It 

 is drilled into children in elementary school and high school so that no 

 one any longer questions the realities of molecules and atoms. When 

 one divides a molecule, it is no longer the same substance; and as 

 soon as one divides an atom, it is no longer the same element. Like- 

 wise, subatomic particles have fixed masses. 



However, the concepts that energy, angular momentum, space, and 

 time come in minimum-size pieces are harder to appreciate. The 

 typical high school science teacher has heard of it but probably can't 

 explain it. Even many elementary physics courses at the college level 

 pass over these ideas as quickly as possible. Yet these ideas are no more 

 surprising or unusual than the existence of atoms and molecules. 



Quantum mechanics arose because of an apparent duality of nature — 

 both electrons and radiant energy seeming like waves in some experi- 

 ments, and in others, like particles. For example, classical experiments 



