EXCITATION 59 



assume that the first complex organic ring structure (such as porphyrins) 

 might have been formed in the atmosphere of the primitive earth by- 

 brush discharges. 



In photochemistry it has long been recognized (9) that it is useful to 

 separate the reaction steps which constitute the overall process into 

 primary and secondary reactions. The primary reactions are those 

 which the light-absorbing molecule undergoes immediately after cap- 

 turing a photon. The secondary reactions are thermal steps involving 

 the radicals or other products of the primary reaction steps. Very few, 

 if any (10), chemical reactions are truly simple in nature. With few 

 exceptions, all those which have been analyzed are the result of a com- 

 bination of a number of consecutive and simultaneous reaction steps. 

 Most commonly, these steps are bimolecular reactions between stable 

 molecules, a molecule and a radical, or two radicals. Less frequently, 

 monomolecular reaction steps, consisting of spontaneous rearrangements 

 or dissociations, are involved. In some cases, chiefly the recombination 

 of atoms or small radicals, reaction steps may be termolecular. Reaction 

 steps of higher order than third probably never occur. In radiation 

 chemistry reaction steps may involve ions. 



Chain reactions are of special interest. They are distinguished by 

 high quantum, or ion-pair, yields. During the course of these reactions, 

 some of the reaction steps, in addition to producing the final products, 

 return to the system the radicals or atoms which were formed by the 

 primary process. In this way a single excitation or ionization act may 

 induce the reaction of many thousands of molecules. 



Excitation 



The following review of electronic excitation is chiefly a restate- 

 ment of well-known classical principles. However, in a few in- 

 stances speculations about the nature of specific processes have been 

 introduced. 



If no chemical process such as delayed dissociation or rearrangement 

 takes place, an isolated excited molecule will have a mean life of not 

 less than 10~^ sec, and it will lose its energy of excitation by emitting a 

 photon. Commonly this fluorescent light will have a longer wave length 

 than that of the absorbed radiation. The longer the normal life of the 

 excited state, the smaller will be the absorption coeflficient for the light. 

 For instance, the direct photochemical excitation of an ordinary stable 

 molecule (with a ground singlet state) to an excited metastable state 

 (with a triplet state, which would have a lifetime of about 10~" sec) is 

 so improbable that the coi-responding absorption can be detected only 



