XIII. ACTION SPECTRA AND ABSORPTION SPECTRA 421 



pally in raising the energy levels of electrons of the outer shells of the 

 atoms. The size of the quantum absorbed is equal to the difference 

 in energy between the two levels concerned. 



WTien absorption is by isolated atoms — e.g., atomic hydrogen — 

 there is a relatively small number of sizes of quanta that can be ab- 

 sorbed, because there is a limited number of energy levels that these 

 electrons can occupy, and hence a limited numl)er of possible transi- 

 tions between these levels. As a result the absorption spectra of 

 atoms may consist of a relatively few isolated wavelengths. Where 

 the atoms are combined to form molecules the absorption spectrum is 

 more complex, because the electronic energy levels of the constituent 

 atoms are modified by the mutual relationships between the atoms, 

 and, furthermore, the rotation of the atoms about each other and 

 their vibration with respect to each other increases the number of 

 quantized energy states. The transitions between rotation states 

 and between vibration states correspond to smaller quanta than those 

 associated with changes from one electronic level to another. Because 

 of the large number and variety of differences in energy levels, the 

 absorption spectra of molecules are composed of bands covering 

 ranges of wavelengths instead of the monochromatic lines that char- 

 acterize the absorption spectra of atoms; and in the case of highly 

 complex molecules, such as are encountered in biological systems, the 

 bands may form a continuum over a considerable range of wave- 

 lengths. Examples of such continuous absorption spectra are shown 

 in Figures 1, 2, and 9. With proper conditions, e.g., at low tempera- 

 tures, it is sometimes possible to resolve such spectra into discrete 

 lines or bands corresponding to the individual quantized states that 

 compose them; but, for general purposes, absorption spectra of bio- 

 logical substances may be regarded as being continuous over a char- 

 acteristic range of wavelengths. 



In simpler compounds, a considerable degree of success has been 

 had in relating chemical stmcture and absorption spectra, certain 

 structures corresponding to absorption in particular wavelength 

 regions, as might be expected from the relationship of quantum states 

 and absorption. However, our knowledge in this respect is still so 

 incomplete that exact correlations between structure and absorption 

 cannot always be made, and this is particularly true in dealing with 

 biological substances. 



It is customary to break up into three general divisions the spectral 

 range considered in this chapter: {1) the visible, that region detected 



