148 



Daniel Riobinstein 



permit evaluation of the quantum yield without having to assume an 

 arbitrary value for the extinction coefficient, or without having to know 

 exactly the amount of energy absorbed by the sensitizing pigment. 



This evaluation is based on the following postulated reaction 



Chi ^ Chi* (1) 



scheme: t^t 



^^1 



2 1 



Chl*+A > A + Chi 



kg (3) 



A^+ B > B''^+ A 



The first equation describes the absorption of light and of the pigment, 

 Chi (which may be chlorophyll) to Chi*. The rate of formation of Chi* 

 is equal to the number of absorbed quanta, K I. The rate constant, 



k , for the decay of Chi* includes all Chi* > Chi transitions, except 



the one leading to the formation of the absorption band belonging to the 

 molecule A (eq. 2). In the second equation, it is postulated that the 

 excited molecule, Chi*, reacts with A to form A^. In the third equation, 

 A-'- decays, during both light and dark periods, by reacting with a 

 substrate, B, assumed to be present in excess. 



The rate constant for the decay of A-*- (k3, eq. 3) can be taken 

 directly from the time-trace after cessation of illumination. The steady 

 concentration of Chi* is easily seen to be 



(4) 



(5) 



It is further postulated that the saturation of the absorption change 

 observed at high light intensities, is due to a limited quantity of reactant 

 A, available for reaction with Chi*. 



