151 



Daniel Rubinstein 



Discussion 



In utilizing eq. (10) for the determination of quantum yields and 

 extinction coefficients by extrapolation of the experimental curves to the 

 two axes, accurate knowledge of several variables is required. A 

 "close fit" of the individual points to the straight line is essential for the 

 extrapolation to be meaningfxil. It is essential that the relative value of 

 I be measured by means of a linear detector, such as a photomultiplier, 

 or by utilizing a constant source of light with calibrated neutral density 

 filters. 



In this determination of quantum yield, we assume a particular set 

 of reactions in which ki, k2 and ks were assumed constant and independ- 

 ent of I (eq. l-3y. Also, the conservation law was assumed to apply to 

 A; that is A + A-*- was assumed to be constant. This idealized case may 

 not always be realized, especially at high light intensities. This will be 

 indicated by deviations from the straight line in our plot. For example, 

 a positive deviation might mean that k^ increases with light intensity. 

 Other deviations in the same direction may be caused by increases in the 

 rate of eq. (3) due to a photochemical reaction supplying substrate for 

 the back reaction. These and other deviations can often be revealed by 

 other plots such as those of absorbance vs. time or temperature. 



Any intermediate A which is present in limiting concentration 

 could be detected not only by the absorption of light, but by other methods 

 (eg . electron spin resonance) and the same equation will apply, provided 

 the effect reaches saturation by the same reaction mechanisms. It is to 

 be noted that the plot in fig. 1 does not give the "total" number of 

 absorbed quanta needed to produce one molecule of A^, but that of the 

 quanta absorbed by the "active" pigment participating in eq. (1). The 

 quantum yields calculated from the above plot may be used in the study 

 of light reaction mechanisms in mixtures of "active" and "inactive" 

 pigments, particularly when the "total" quantum yield can be obtained by 

 conventional methods. 



A more detailed study of these relationships will be published 

 later. Thanks are due to Professor Eugene Rabinowitch, Professor 

 Gregorio Weber, Professor Bernard Abbott and Dr. Ashish Ghosh for 

 valuable discussions. 



