226 



RADIATION BIOLOGY 



3-3d. Internal Conversion. ^^ It has been shown that the potential sur- 

 face of a given excited electronic state will usually intersect the surfaces 

 of a number of other electronic states. Predissociation occurs when a 

 molecule, after absorbing a photon and undergoing a transition from the 

 ground state (A) to some vibrational level of some excited electronic state 

 {B), subsequently undergoes a radiationless transition to another excited 

 electronic state (C) in a region where the vibrational excitation energy of 

 C exceeds its dissociation energy (and its vibrational "levels" are there- 

 fore continuous). It may also happen, however, that the potential sur- 

 face of B intersects the surface of a state D in a region where the vibra- 

 tional excitation energy of D is smaller than its dissociation energy. In 

 this case a radiationless transition may occur, but dissociation of the mol- 

 ecule will not result. Since the electronic excitation energy of D will 

 ordinarily lie below that of B, there has occurred a conversion of part of 

 the electronic excitation energy of a molecule into vibrational energy by 



a purely internal process. If the state 

 D happens to be the same as ground 

 state A, all the electronic excitation 

 energy will be so converted. The 

 probability that such a transition will 

 reverse itself is very small, for the vibra- 

 tional energy, initially concentrated in 

 a few degrees of freedom, will swiftly be 

 dissipated to many of the degrees of 

 freedom of the molecule, and to those 

 of other molecules in subsequent im- 

 pacts. Thus the process of internal 

 conversion is, from the physical point 

 of view, a facile mode of direct trans- 

 formation of electronic energy into 

 heat. From the chemical point of 

 view, internal conversion is a process 

 whereby activation is followed uni- 

 deactivation rather than by decomposition (as in 



INTERATOMIC DISTANCE 

 Fig. 3-8. Potential curves to illus- 

 trate the improbability of internal 

 conversion in a diatomic molecule 

 (schematic). 



molecularly by 

 predissociation). 



The conversion of electronic energy into heat (motion of atoms) is a 

 process which in a system composed of isolated atoms is exceedingly 

 slow (cf. Sect. 3-2c). In a diatomic molecule the process is again of so 

 small probability as to be of no consequence. This may be concluded 

 from a simple schematic example (Fig. 3-8). A radiationless transition 



13 Internal conversion in polyatomic molecules has no connection with internal 

 conversion in excited nuclei, although there is an important formal relationship m 

 that in both cases a radiationless transition occurs between a discrete excited state of a 

 system and that state of equal energy in a continuum of states of completely different 

 physical nature. 



