420 



HAROLD F. BLUM 



B. ABSORPTION SPECTRA 



An absorption spectrum represents the absorption of light by a 

 given substance with respect to wavelength or frequency. Examples 

 are shown in Figures 1, 2, 5, and 9. The absorption spectrum reflects 

 the chemical composition of a substance, being determined both by 

 the kind of atoms of which the substance is composed and the spatial 

 and energy relationships of these atoms. Thus, each chemical com- 

 pound has a characteristic absorption spectrum, which may be used 

 oftentimes to identify the compound or its general type, since similar 



280 400 



WAVELENGTH, m/i 



Fig. 1. Absorption spectra of carot- 

 inoid pigments (20a). The "specific 

 absorption coefficient" is t/molecular 

 weight. 



200 400 600 



WAVELENGTH, m^ 



Fig. 2. Absorption spectra of some 

 porphyrins {2). Plot of log k' per- 

 mits ready comparison of shapes of 

 curves. 



molecules may have similar absorption. The presence of certain radi- 

 cals in large molecules, and their position in the molecule, may be re- 

 flected in the absorption spectrum, helping to determine chemical 

 structure. 



The results of absorption of quanta of radiant energy may be quite 

 different, depending upon the size of the quantum and the nature of 

 the absorbing material. The effects of the large quanta constituting 

 ionizing radiation, which may cause electrons to be removed from the 

 atom or molecule thereby forming ion pairs, have been treated in 

 other chapters and will not be discussed here. This chapter is con- 

 cerned with the absorption of small cjuanta, which results princi- 



