ULTRAVIOLET ABSORPTION SPECTRA 177 



plane along either of two mutually perpendicular axes (.r, y), producing 

 different molecular orbitals. If the molecule is asymmetric (as in naph- 

 thalene), these orbitals will represent different energy levels, and transi- 

 tions from the ground state to these orbitals will be associated with spec- 

 troscopically distinct absorption bands, which will be strongly dichroic 

 (Coulson, 1948). The intersections of the nodal planes with the molec- 

 ular plane for the molecular orbitals of anthracene are indicated in Fig. 

 5-9. The transitions that correspond to the two prominent ultraviolet 

 absorption bands of anthracene are labeled with the band wave length. 

 It can be shown that these two approaches, valence-bond and molec- 

 ular-orbital, in their simple forms, probably bracket the correct solution; 

 the valence-bond method does not allow enough weight to possible ionic 

 structures, i.e., those in which two or more of the bonding electrons may 

 be concentrated on one atom; the molecular-orbital method allows too 

 much. More advanced developments of both theories have tended to 

 narrow the gap between them. 



SPECIFICATION OF ABSORPTION 



INTENSITY 



The ultraviolet absorption spectra of substances of biochemical interest 

 are usually obtained with solutions of these substances in transparent 

 solvents. The measured absorption at any wave length will then be 

 dependent on the concentration of the substance in the solution and the 

 length of the light path in the solution. The specification of absorption 

 spectra may be standardized by referring all measured spectra to the 

 spectrum that would be obtained from a solution of a standard concentra- 

 tion and a standard light path. This conversion of measured to standard 

 spectrum is rendered easy by the simple nature of the formula relating 

 absorption to concentration and light path. 



Since the absorption of a photon by a molecule is an all-or-none act and 

 since all molecules may be assumed to have, statistically, the same proba- 

 bility of absorption of an incident photon of a given wave length, any 

 layer of a solution of thickness dl, transverse to the light beam, may be 

 expected to absorb the same fraction of radiant energy of one wave length 

 as any other such layer, and if dl is small, this fraction will be proportional 

 to dl. Thus 



jT ^ dt. 



This statement is known variously as Lambert's or Bouguer's law. 



If the absorbing molecules may be assumed to act independently, the 

 fraction of incident energy absorbed in a given layer will be expected to be 

 proportional to the concentration of absorl)ing molecules in the solution. 



