274 BIOLOGICAL EFFECTS OF RADIATION 



i.e., restricted to definite integer multiples of a quantum, because the 

 molecule dissociates in less time than is required for rotation. Since 

 the kinetic energy of the fragments is not quantized, the energy absorbed 

 from the spectrum extends over a slightly larger range of frequencies. 

 Examples are found in the spectra of sulfur, ammonia, nitrogen dioxide, 

 and other molecules. 



Intensity. — In a simple primary photoprocess the amount of photo- 

 chemical reaction should be directly proportional to the amount of light 

 absorbed. If Einstein's relationship holds and one molecule reacts for 

 each quantum absorbed, the extent of the reaction will depend only on 

 the number of quanta absorbed and this in turn will depend directly 

 upon the amount of light absorbed. This relationship is found to hold 

 in a great many photochemical reactions. In many other photochemical 

 reactions, however, the amount of chemical reaction is not directly 

 proportional to the amount of Hght absorbed, and this deviation from 

 direct proportionality often provides a clue as to the mechanism of the 

 photochemical reaction. Several factors may be responsible for deviation 

 from this simple proportionality. In some cases the molecules of absorb- 

 ing material are broken up into atoms by the absorption of light and 

 the atoms take part in the reaction. Then two atoms may be formed 

 for each quantum absorbed and the concentration of reacting materials 

 is twice as great as predicted on the basis of the Einstein relationship. 

 "Under these conditions the application of the mass law shows that the 

 amount of reaction varies as the square root of the light intensity rather 

 than as the intensity itself. Sometimes a secondary thermal reaction 

 follows the primary photoprocess, but when the light intensity becomes 

 very great, the primary process may become so rapid that the secondary 

 thermal reaction cannot keep pace. Again, at high intensities the 

 reacting materials for the primary reaction may be so greatly depleted 

 as to slow down the reaction. A classic example of this situation is found 

 in photosynthesis, with chlorophyll, in the so-called Blackman region 

 where at high intensity the photosynthesis is proportionately less than 

 at low intensities because the carbon dioxide material cannot diffuse 

 rapidly enough into the cells; e.g., the supply of carbon dioxide rather 

 than the supply of photons becomes the hmiting factor. The high inten- 

 sity of light may also produce a congestion of reaction products in such 

 a way that the reaction is slowed down— or perhaps accelerated. Effi- 

 cient stirring becomes specially important under these conditions. 



Length of Exposure.— In a simple, primary photochemical process 

 the amount of chemical change should be directly proportional to the 

 time of exposure, provided that the quantity of light absorbed is the same 

 throughout the exposure. If such is not the case, the reaction is shown 

 to be complex. Several complications may account for the failure to 

 obtain proportionality, such as, autocatalytic effects, the destruction of 



