12 



RADIATION BIOLOGY 



delayed by the prohibition of radiational transitions between states of 

 different multiplicity. Either the transition probability to the ground 

 state will be low (Fig. 1-6, path 2), or an activation energy £"0 is required 

 to restore the system to the original surface from which radiation is 

 allowed (Fig. 1-6, path 3). Temperature dependences will frequently 

 distinguish which situation exists. The phosphorescence of anthracene, 

 biphenyl, and thiobenzophenone appear to be thus explained. On the 

 other hand, the state to which crossing occurs may belong to a tautomer 

 of the original molecule (Franck and Livingston, 1941). For example, 



one or more hydrogen atoms of 

 chlorophyll may take new positions 

 during the crossing process to form 

 a new chemical species, though mod- 

 ern evidence favors a triplet meta- 

 stable state (Franck, 1951). If the 

 ground state of the new species lies 

 higher than the ground state of the 

 original form, some of the excitation 

 energy is stored as the additional 

 free energy of formation of the tau- 

 tomer. The transition from C to 

 ^4 may be forbidden because (1) it 

 involves a change in multiplicity or 

 (2) curve C may actually cross A 

 and so be the lowest surface for the 

 configuration corresponding to the 

 minimum in C. When reversion to 

 the original molecular form in its 

 excited state is restricted by an ap- 

 preciable activation energy, phos- 

 phorescence can be an important final result, as Franck and Livingston 

 (1941) have proposed. The phosphorescence of trypafiavine under spe- 

 cial conditions has been explained in this way (Kautsky et al., 1933; 

 Weiss, 1935). Internal rearrangement of nuclei is probably important 

 in many photoinduced reactions. Several of the reactions of chlorophyll 

 in vitro have been satisfactorily explained in this way, but the interpre- 

 tations are not unequivocal. 



Generally speaking, the consideration of single excited levels greatly 

 oversimplifies the picture. In Sect. 1-4, the number of surfaces for n 

 unexcited atoms was given as w!/(n!/2!)-. In the absence of symmetry 

 in the molecule, this is the number of different energy levels. If n is 16, 

 there will be 3 X 10^ potential-energy surfaces corresponding to unexcited 

 atoms. If all these states lie within 100 ev (2300 kcal) of the ground 

 state, the levels will be about 3.3 mv, or 76 cal, apart. Very nearly a 



INTERNUCLEAR DISTANCE 



Fig. 1-6. Potential-energy curves illus- 

 trating internal conversion to a meta- 

 stable state C, triplet, or other tautomer. 



