92 



cule. At any rate, certainly by recombination of the radicals and electrons as 

 well as by direct electron excitation one should obtain a good yield of triplet 

 states. 



The observation has been made by the Notre Dame group that aromatic 

 molecules tend to protect aliphatic molecules with respect to radiation decom- 

 position. I once pointed out that this type of process may be explained by triplet 

 state formation. If irradiation leads to an aliphatic radical ion, A+, then charge 

 transfer may occur to the aromatic molecule B, in the system, giving A + B + . 

 The B + ion would then combine with the electron to give stable excited states, 

 triplet, or singlet. If recombination occurred between the aliphatic A + and the 

 electron, repulsive states would occur and bond rupture result. 



BURTON: At the present time I may say that I no longer advocate that par- 

 ticular mechanism. I think it is more likely to be an excited molecule mechan- 

 ism rather than an ionic one. 



LINSCHITZ: In any case even if energy rather than charge is transferred, 

 this would give you a triplet molecule which in the aromatic case is stable 

 whereas in the aliphatic case it is unstable. This conversion will bleed the en- 

 ergy off as heat. 



That essentially covers what I wanted to say. 



FANO: I would like to elaborate a little on a point I made earlier. It has 

 come out from subsequent discussion with Dr. Linschitz that the question he 

 had in the back of his mind was what fraction of the energy delivered by ionizing 

 radiation goes into excitation, and now the question is, what is meant by excita- 

 tion. Let's try to distinguish just roughly between energy transfers to molecules 

 above and below something like 12 to 15 electron volts. Whatever is above is 

 probably what I would naively call ionization and below 12 to 15 volts would be 

 what I would naively call an excitation. But probably excitation in the meaning 

 of Dr. Linschitz and of photochemists in general is really something considerab- 

 ly lower, say below 5 volts which is the sort of thing which is known to be taken 

 up by the chromophoric group in photochemical reaction. So maybe one could 

 ask first how much is below the 5 volt limit, how much is above the 12-15 volt 

 limit, and how much is in between. I would say the primary criterion for trying 

 to answer a question like this is what portion of the total oscillator strength lies 

 in these different ranges, and I would say I don't have any information, but I 

 would be inclined to say that probably the ratio of above to below the 12-15 limit 

 for any sort of ordinary material is of the order of half and half. I am just tak- 

 ing it from hydrogen, but that must be quite general. 



Incidentally, this fraction depends on the ionization limit in a queer manner. 

 The higher the limit the more probable it is that the energy absorption goes 

 above it. It is just contrary to what one might expect. 



When it comes to saying how much of the energy goes below the 5- volt limit, 

 then I would say that this is probably a very small fraction. It is a small frac- 

 tion for the reason that the percentage of electrons in the material which can ab- 

 sorb such low amounts is small. If many of the electrons in organic matter were 

 ring electrons of aromatic molecules, then the great majority of the energy will 

 go below 5 volts. But the percentage of electrons in the material which belong 

 in aromatic rings or any how to double bonds is extremely small. I haven't 

 made any calculations and I don't know, but I would say probably a few per cent. 



