60 REACTIONS INDUCED BY IONIZING RADIATION 



under quite special conditions. The transition probabilities for both 

 absorption and emission are determined by selection rules and by the 

 Franck-Condon principle. For large or even moderately complex 

 molecules the selection rule which holds most generally is that which 

 "forbids" transitions between states of different multiplicities. It is, 

 for instance, the reason that a transition from a singlet to a triplet state 

 is 10"^ times less probable than otherwise similar transitions which do 

 not involve changes in multiplicity. The Franck-Condon principle 

 states that during an electronic transition the positions of the heavy 

 nuclei and their kinetic energies practically cannot change. Thus the 

 absorption spectrum permits conclusions about the position of the 

 nuclei at the moment of the absorption act. The heavy nuclei can, how- 

 ever, gain potential energy by the electronic transition, and will there- 

 fore start to oscillate if their equilibrium position in the excited state is 

 different from that in the ground state of the electronic system. Usually 

 excitation weakens the binding energy between nuclei and thereby 

 enlarges their equilibrium separation. If this difference is sufficiently 

 great, as it is for the hydrogen iodide molecule, an application of the 

 Franck-Condon principle shows that photochemical excitation results 

 almost exclusively in the formation of an electronically excited molecule 

 with oscillational energy greater than its energy of dissociation. Ac- 

 cordingly, it will dissociate after a single vibration (about 10~^^ sec) into 

 radicals. Under these conditions the gas is, of course, non-fluorescent 

 and photochemical reactions are very probable. 



A process called predissociation (1) is of importance for complex 

 molecules, including moderately complex compounds, such as nitrogen 

 dioxide. Whenever two electronic states of a molecule have the same 

 nuclear configuration and total energy, there is a finite probability that 

 a molecule which is in one of these states will "cross over" into the other. 

 The value of this probability depends on certain selection rules and upon 

 the time the molecule spends in the configuration which is common to 

 both states. As a result of this process, a molecule, in an excited state 

 in which it does not have enough oscillational energy to dissociate, may 

 after a lapse of time cross over into a second excited state in which it is 

 unstable. This second state either may be less stable than the first or 

 may be a completely repulsive state. Depending upon the probability of 

 transfer, the mean life of the excited molecule may be anything between 

 the period of a single vibration (10~^^ sec) and the normal life of the 

 excited state (10~^ sec). The occurrence of predissociation is marked 

 by the appearance of photochemical action, by a decrease in the intensity 

 of fluorescence, and by the disappearance of the rotational structure of 

 the absorption bands. 



