634 



ABSORPTION SPECTRA OF PIGMENTS IN VITRO 



CHAP. 21 



(21.1) 



/-C 



0.24 per molecule chlorophyll a (in ethanol) 

 22 per molecule chlorophyll b (in ethanol) 



In the quantum theory, / is the measure of the transition probability 

 between the states Xo and Fo; and the reciprocal of the transition probabil- 

 ity is the mean life-time, t, of the excited state (as far as the latter is limited 

 only by the fluorescent transition Fo -^ Xo) . The relation between / and 

 r is: 



(21.2) 



3 ^, mc 



8 e^ir 



2 1 _ 1.96 X 10- 



where m, c, e and tf have the usual meaning. The theoretical mean life- 

 time of chlorophyll molecules in the lowest excited state (reached liy ab- 

 sorption of red light) is therefore : 



(21.3) 



= { 



8.2 X 10~8 sec. for chlorophyll a (in ethanol) 

 8.9 X 10~* sec. for chlorophyll h (in ethanol) 



The higher intensity of the blue-violet absorption band (particularly in 

 chlorophyll b) indicates that the natural life-time of the excited state B, 

 is somewhat shorter than that of state F — probably 5 X 10 ~^ sec. or less. 



NUCLEAR DISTANCE 

 Fig. 21.22. Crossing of potential curves. 



The actual life-times of chlorophyll in states B, A and F are considerably 

 shorter than the "natural" life-times. This is indicated by the complete 

 absence of fluorescence originating in the levels B and A and the relative 

 weakness of fluorescence originating in level F. 



From the complete absence of blue-violet fluorescence in chlorophyll 

 solutions (c/. page 748) it follows that the energy of state B must be dissi- 

 pated within 5 X 10 ~'^ sec. or less. (With the natural life-time of 5 X 



