SOME PHOTOCHEMICAL CONSIDERATIONS 15 



According to the Einstein law, each molecule is excited by the absorption of 

 one quantum, so that 



<p = 1 



This law is applicable only to a primary photochemical reaction. Secondary 

 dark reactions may mask the true value of the quantum yield, as the following 

 examples show. 



In the primary reaction of the photochemical dissociation of hydriodic 

 acid 



HI + //J/ ^ H + I 



the quantum requirement \ I ip = 1. This primary photochemical reaction 

 is immediately followed by two secondary dark reactions 



H + HI -^ H., + I 



and 



I + I -^ I2 



The sum of the primary photochemical and the two dark reactions is repre- 

 sented by 



2HI + hu -^ Ho + I.> 



It follows from this over-all reaction that the quantum requirement l/<p = 

 0.5. 



The photochemical decomposition of ammonia is an example of a reaction 

 with \/<p > 1. The primary photochemical reaction 



NH, + hv -^ NHo + H 



is followed by several dark reactions, e.g., 



0.8 NH. + 0.8 H -^ 0.8 NH, 

 0.2 NH. + 0.2 H -^ 0.2 NH + 0.2 Ho 

 0.2 NH -* 0.1 N,. + 0.1 Ho 



The sum of the photochemical and the three dark reactions is 



0.2 NH., + liv -^ 0.1 No + 0.3 H. 



so that the quantum requirement is l/> — 5. 



These examples show that the quantum requirement of photochemical 

 reactions may deviate considerably from the theoretical value owing to 

 secondary dark reactions. Therefore, when it is found that a process occurring 

 with absorption of radiation energy has a quantum requirement which is not 

 equal to 1, it must be assumed that dark reactions are involved. The 

 complex of reactions must then be disentangled until the correct primary 

 photochemical reaction with l/V = 1 is detected. In our example of the 

 photochemical decomposition of NH3 the quantum requirement of the 

 over-all reaction would have other values if the experimental condidons were 



