67 



ALLEN: This was such a concentrated peroxide solution that the amount 

 of peroxide was comparable to that of water in the solution. 



BURTON: In such a case it is very easy to have a typical Stern-Volmer 

 reaction and to have the excited molecules react with the hydrogen peroxide. 

 I don't know how you establish the fact that it is a radical pair yield. 



HOCHANADEL: Dainton and Rowbottom (1 5) determined the radical pair 

 yield for gamma rays in liquid water by comparing the radiation yield in the 

 radiolysis of carefully purified hydrogen peroxide with the quantum yield in 

 the photolysis of the same solution. Assuming a primary quantum yield of 

 2.0 for dissociation of hydrogen peroxide into two hydroxyl radicals and also 

 assuming that in both radiolysis and photolysis the rate is proportional to the 

 square root of radiation intensity they calculated a yield of 13.4 radical pairs 

 per 100 ev. This value is much higher than previously reported values which 

 ranged around three radical pairs per 100 ev. 



BURTON: Doesn't this experiment assume what is intended to be 

 proved? It assumes that all the decomposition of the hydrogen peroxide you 

 get is a result of a primary radical formation. Then Dainton calculates 

 what the primary radical formation is in the case of the gamma-irradiated 

 case on the basis of hydrogen peroxide that has been decomposed. However, 

 there is an essential difference between gamma-irradiation and photon irradi- 

 ation. In the case of photon irradiation, we deal practically exclusively with 

 optically allowed transitions. Consequently, there is a good possibility that 

 the only way the excited state so produced can make a contribution is that it 

 decompose to give radicals; otherwise the excited states may disappear by 

 fluorescence, for example, before anything else may happen. On the other 

 hand, in the case of gamma irradiation a number of optically forbidden 

 transitions may occur. Thus, the excited states may be quite resistant to 

 emission of light as a method of degradation of energy. They may conse- 

 quently survive sufficiently long to enter into radio-sensitization or straight- 

 forward metathetical reactions. Consequently, for the very high hydrogen 

 peroxide concentration that I understand Dainton was employing there is a 

 very high probability of reaction between excited water molecules and hydro- 

 gen peroxide molecules. It does not follow that his results really mean that 

 he had this high radical pair yield. I don't say it is wrong. I simply say 

 it does not necessarily follow from his work that he had these high radical 

 pair yields. 



ALLEN: I think the way I would put your point is that you may get ex- 

 cited states formed in the irradiation of water which normally disappear 

 without giving any chemical effect. But if you have a fairly concentrated 

 solution of some quite unstable molecule, such as hydrogen peroxide, the 

 energy of these excited states may pass over to the hydrogen peroxide by 

 excitation transfer, leading to its decomposition. 



BURTON: Or have a Stern-Volmer reaction, but in either case under 

 those conditions the excited H^O is sufficiently persistent for one of those 

 two reactions to happen. 



LINSCHITZ: I don't know whether there are any stable, long-lived ex- 

 cited states of water at all. 



MAGEE: I don't know of any experimental work on this. I know there 

 is a paper by Niira (16), who calculated some low-laying electronic states of 

 H^O. He says that the HOH molecules, in a linear form, in a triplet state, 



