136 ILLINOIS STATE ACADEMY OF SCIENCE 



RECENT DEVELOPMENTS IN PHOTO- 

 CHEMISTRY 



W. Albert No yes, Jr., University of Chicago 



The advent of the quantum theory has given us a means 

 of applying some of the principles of thermodynamics to 

 photochemical reactions. According to this theory, the 

 energy of frequency v incident upon a given system is 

 necessarily some multiple of an energy unit, ' hv . The 

 Bohr theory of atomic structure has successfully applied 

 this idea to the spectrum lines of hydrogen and to X-ray 

 data. According to this theory there are several pos- 

 sible "energy states" or "stationary states" for an 

 atom. When the atom is in its normal state it does not 

 radiate energy, but if energy is added to the atomic sys- 

 tem either by electron impact or by radiation, the total 

 energy of the system is now greater than for the normal 

 atom by a definite amount. This energy may be repre- 

 sented by E . If now the atom returns to its original 

 energy state, light of frequency given by the simple 

 equation E = hv is radiated, where h is Planck's con- 

 stant. 



Several attempts have been made to apply the ideas 

 of the quantum theory to photochemical processes. Ein- 

 stein 1 has derived a photochemical equivalence law, 

 which, briefly stated, equates the radiant energy neces- 

 sary to cause a mol of substance to react to the heat of 

 reaction 



AH = Nhv 

 In the case of a system A' capable of changing by the 

 action of radiation of frequency v into a system A', 

 radiation of frequency v' being re-emitted in the process 

 according to the scheme 



A + hv — KV + hv' 

 then the heat of reaction would be given by the expres- 

 sion 



AH = Nh(v— v') 

 It should be noted that radiation of frequency V would 

 cause the reverse reaction to take place if it were al- 

 lowed to act on the system A'. 



1 Einstein, Annalen der Physik, 57, 832 (1912). 



