218 



RADIATION BIOLOGY 



3-2f. Predissociation. A diatomic molecule in an excited electronic 

 state having a stable potential curve may, under certain circumstances, 

 dissociate even though its vibrational level lies below the dissociation 

 limit of that state. This phenomenon, in contrast to direct optical 

 dissociation, does not take place immediately upon transition to the 

 excited state, or more precisely, in a time of approximately one vibra- 

 tional period thereafter; rather, it requires many — 10- or 10^ or even 

 more — such periods. It is found to occur only if the potential curve of 

 the excited state crosses, in particular manner, the curve of another 

 electronic state, and is known as predissociation — the somewhat mislead- 

 ing designation given it when first discovered. 



Examination of Fig. 3-3 shows that for several excited states of H2 all 

 vibrational levels of the excited state lie higher than the dissociation 

 limit of the ground state. Such a situation might raise the question of 

 why the excited molecules could not dissociate spontaneously by passing 

 over to a vibrationally and rotationally excited level of the ground 

 electronic state which would have the same total energy as the original 

 state, and would therefore dissociate at once. The Franck-Condon 

 principle provides the answer: such a transition would require a sudden 

 and relatively great change in interatomic distance and thus is not 

 possible. 



In Fig. 3-6, which is schematic, the potential curve of excited state B 



intersects that of excited state C. If 

 the state B is formed, say by light ab- 

 sorption, but with vibrational energy 

 below that corresponding to the inter- 

 section point (transition I in the figure), 

 a dissociation, although it may be ener- 

 getically possible, is still forbidden by 

 the Franck-Condon principle. If the 

 state B has vibrational energy in excess 

 of the afore-mentioned amount (transi- 

 tion II in the figure), however, in its 

 vibration it will continually pass through 

 a stage in which its interatomic distance 

 is equal to that for the intersection 

 point. At this stage a transition, or 

 " crossing over" from state B to state C, 

 is permitted both by energy conserva- 

 tion and momentum conservation (Franck-Condon principle). It would 

 therefore have a certain definite probability of occurring; or, in other 

 words, an excited molecule B will have a definite Ufetime for predissocia- 

 tion. If that lifetime is much smaller than the time for all other processes 



ABC 



ABSORPTh 



INTERATOMIC DISTANCE 

 Fig. 3-6. Potential curves to illus- 

 trate one type of predissociation 

 (schematic). 



