2 RADIATION HIOLOGY 



(hen uiulergc) (or initiate) a series of siinplc thermal reactions, the second- 

 ary steps. Secondary reaction steps arc imiinolecular, himolecular, oi- 

 termolecuhir reactions. ( )1 these, himolecuhir reactions, which involve 

 a radical or atom and a stable molecule, are probably the most important. 

 Bimolecular reactions also occur between two radicals or atoms. In the 

 gas phase, recombination of two atoms always occurs as the result of a 

 three-body collision. This is very probably the only important type of 

 termolecuhir reaction step. Unimolecular reaction steps are limited 

 to the "spontaneous" decomposition or rearrangement of comple.x 

 molecules. 



PRIMARY STEPS 



ABSORPTION 



Each photon is absorbed by a single molecule,' and, under all ordinary 

 conditions, multiple e.xcitation of a molecule by successive capture of two 

 or more photons is of negligible importance. By capture of a photon the 

 molecule is raised from the ground state to a higher electronic state. 

 With few exceptions (notably molecular oxygen and "odd" molecules; 

 Herzberg, 1950), the ground state of a stable molecule is a singlet one, 

 and the molecule will be excited preferentially to a higher singlet level. 

 The selection rule, which "forbids" transitions betw^een states of different 

 multiplicities, is called the "intercombination rule." For ordinary mole- 

 cules the probability of a transition between energy levels of unlike multi- 

 plicity can be 10^-fold less than that for an otherwise similar transition 

 which does not violate the rule. This is one of the few selection rules 

 which apply to complex molecules as well as to atoms and diatomic mole- 

 cules. For certain cases, such as those involving heavy atoms, e.g., mer- 

 cury, the rule applies less strictly, and the probability of transition is 

 reduced by a factor of only about 10'*. 



FRANCK-CONDON PRINCIPLE 



The Franck-Condon principle is probably of greater importance than 

 the selection rules in determining the photochemical behavior of mole- 

 cules. This principle states that electronic transitions which involve 

 appreciable changes in the positions or momenta of the constituent 

 nuclei have relatively low probabilities.- At ordinary temperatures, 

 molecules which exist in thermal e(|uilibrium with their surroundings are 

 in their lowest oscillational states. Accordingly, the Franck-Condon 

 principle and the potential-energy curves determine the oscillational 



' In this disrussion, certain typos of crystals and crystalline micelles are to be 

 regarded as single molecules. 



^ This discussion of the Franck-Condon principle is based on its original classical 

 formulation. For a discussion of the modifications introduced by quantum theory see 

 Herzberg (1950). 



