CHAPTER 2 



Some Photochemical Considerations 



§ 5 The Einstein Law 



In a photochemical reaction a molecule or an atom is brought from its 

 normal ground state to an excited state by the absorption of radiation energy. 

 In 1911 Einstein, encouraged by E. Warburg, extended the quantum theory 

 to photochemistry. According to his law of photochemical equivalence, each 

 molecule is excited (activated) by the absorption of one quantum. Thus, 

 one mole (A^ molecules) is excited by the absorption of A^ quanta, A^ being 

 the Avogadro number (A^ = 6.02 X 10-'). We may consider A^ quanta to 

 to be one mole quanta, just as we may say that A' electrons are one mole elec- 

 trons. As one mole electrons is called the electrochemical equivalent (1 

 faraday), it is customary to say that one mole quanta (1 einstein) is a photo- 

 chemical equivalent. It must be borne in mind that the law of photochemi- 

 cal equivalence is not an arbitrary hypothesis but, as its name indicates, a 

 thermodynamically well-founded law. 



The energy amount of a photochemical equivalent is : 



E = Xhv 



Planck's constant // has the value 6.626 X 10"-' erg-sec and v is the frequency 

 of the radiation expressed in sec^^ It follows from the theory of radiation : 



\v = c 



where c is the velocity of light (2.998 X lO^" cm-sec-^) and X is the wave-length 

 in cm. Hence, we find 



or 



or 



^ 6.02 X W-' X 6.626 X IQ--' X 2.998 X IQi" , , 

 E = — erg/ mole 



^ 1.197X10^ / , 

 E = r erg/ mole 



A 



or, 1 erg being 0.239 X 10-' cal 



Z7 2.86 ,, , 

 E = ^T— cal/mole 



