1472 



PHOTOSYNTHESIS IN INTERMITTENT LIGHT 



CHAP. 34 



curves predicted flash saturation only at flash energies of the order of 10* 

 lux sec, instead of < 10^ lux sec. as observed by Tamiya. 



A different kinetic postulate was therefore tried by Tamiya — that the 

 back reaction is of the second order. In scheme 28.11, this would be the 

 case if the two photoproducts, AHCO2 and A'HO, were to become kineti- 

 cally independent before reacting back ; bimolecular secondary back reac- 

 tions were provided (in addition to monomolecular primary back reactions) 

 also in schemes 28.IA and 28.IB (pp. 1025-1026; reactions 28.20d and 

 28.21d). 



The postulate of a bimolecular back reaction naturally leads to a more 

 sudden approach to flash saturation (since the rate of the back reaction now 



(a) 



(b) 



d. 6 



o 

 E 4 



0.10 0.20 



td , sec. 



0.30 



200 400 600 



/, tux sec. 



Fig. 34.25. Interpretation of Tamiya and Chiba's results by a second-order back reaction 

 (after Tamiya 1949): (a) P = f{td) curves; (b) P = /(I) curves. 



increases with the square of the concentration of the photoproducts). 

 Kinetic equations of somewhat forbidding complexity were derived by 

 Tamiya for this mechanism. The rate constants (/cie, kie, and the bi- 

 molecular back reaction constant, kg) were calculated, by means of these 

 equations, from the same experimental parameters as was done before for 

 a first-order mechanism; a fairly good representation could be obtained 

 this time not only for the flash yield vs. dark time experiments, but also for 

 the flash yield vs. flash energy curves (fig. 34.25). 



Tamiya suggested that the results of Emerson and Arnold could be ac- 

 counted for quantitatively by the same equations, if one assumed that the 

 flashes used by them had an integrated energy of about 100 lux sec. (cf. 

 fig. 34.22). However, Tamiya's explanation requires the assumption that 

 the flash yield of Emerson and Arnold was measured in the hnear range of 

 the yield vs. energy curve — which is incompatible with the experimental 

 data, clearly showing an approach to (if not attainment of) saturation. 

 Fig. 34.27 (p. 1479) of Ehrmantraut and Rabinowitch (1952), obtained Math 

 a higher discharge energy, shows even more clearly than the data of Emer- 



