224 RADIATION BIOLOGY 



coincides with the long-wave-length limit of strong absorption. The 

 intensity distribution of the fluorescence band extending toward longer 

 wave lengths in these cases mirrors to a first approximation the absorption 

 band which extends toward shorter wave lengths. This relation obtains 

 if the atomic configuration in the ground state and that in the lowest 

 excited state are so similar that very fittle energy transfer to oscillational 

 energy (via potential energy) is coupled with the electronic transition. 

 Many dyestuffs containing ring systems of conjugated double bonds 

 belong to this class of molecules. 



In other cases, for example in saturated organic compounds, the equi- 

 hbrium configuration of the atoms in the excited state is markedly differ- 

 ent from that in the ground state. In these cases the oscillational energy 

 must increase greatly in an electronic transition. The relation between 

 fluorescence and absorption spectrum is then no longer as simple as for the 

 other class of molecules. If, for instance, excitation is caused by the 

 absorption of light of the long-wave-length limit of the first absorption 

 band, the atomic constituents gain so much potential energy that they 

 start to oscillate violently. It will take a long time before all the atoms 

 with different oscillational frequencies find themselves momentarily again 

 in the configuration which the molecule had at the moment of the light 

 absorption act. Even if the molecule is not subjected to impacts, the 

 time needed to reach this particular configuration may be much longer 

 than the lifetime of the excited molecule. Thus the spectral region of the 

 first absorption band may have much shorter wave length than the region 

 of the fluorescence band. Shifts in wave length by a factor of two or more 

 are by no means rare. 



Similar considerations play a role in collisions of all types, whether they 

 result in transfer of energy or an electron, or in chemical change. 

 The system must follow continuously the potential surface of the molec- 

 ular (activated) complex which includes both partners of the collision. 

 A transition to the surface of a different electronic state becomes possible 

 only if, by internal motion of the atoms in the complex, an intersection 

 of the two surfaces is reached. 



3-3c. Dissociation and Predissociation. If a polyatomic molecule 

 absorbs a photon and undergoes a transition to an excited electronic state, 

 and if in this state the total vibrational energy in all modes which the 

 molecule acquires as a result of the transition (Sect. 3-3b) exceeds the dis- 

 sociation energy for any one vibrational degree of freedom, there must 

 always be a nonzero probability that the molecule will dissociate. The 

 magnitude of this probability depends on the time required to reach a con- 

 figuration corresponding in the particular degree of freedom to dissocia- 

 tion of a diatomic molecule (location on a repulsive region, etc.), or, in 

 other words, on the time required for a sufficient portion of the fluctuating 

 energy to be concentrated there, and also on the required periods for com- 



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