95 



with X rays under the same conditions, deactivation is irreversible. I presume 

 this will hold for betas and alphas. 



POLLARD: Incidentally, only about 70 per cent is reactivated, so there is 

 30 per cent that holds out. 



KAMEN: But there is a qualitative difference between ionizing radiation and 

 ultraviolet. 



BURTON: One of the things that one must never forget is that in absorption 

 of ultraviolet radiation the absorption is very specific. On the other hand, for 

 ionizing radiation the absorption is completely non-specific. 



KASHA: May I make one comment appropos to the remarks on water and its 

 triplet state? One would be interested in seeing whether any picture can be 

 given for a stable (excited) triplet state of the water molecule. There are two 

 sigma (axially symmetrical binding) orbitals in this molecule. Each such orbit- 

 al contains two electrons, one of which could be excited. But a sigma electron 

 could be excited only to a sigma anti-bonding orbital, which would result in bond- 

 rupture, or to a Rydberg orbital, which might result in ionization . However, 

 the water molecule contains in addition several essentially non-bonding electrons 

 on the oxygen atom. One of these latter n- electrons could be excited to an anit- 

 bonding sigma orbital. The excited state could be either singlet or triplet in 

 such a case, and would probably be stable, i.e., the molecule would not disso- 

 ciate upon excitation. This is one interpretation of a possible stable excited 

 triplet state of the water molecule. Such a triplet state might lie in the near 

 ultraviolet, in contrast to the normal transitions of the water molecule which oc- 

 cur in the vacuum ultraviolet. 



LINSCHITZ: The lifetime would be very long for such a molecule, that is 

 clear. 



KASHA: Yes. 



PLATZMAN: I have a remark to make about the triplet states -- not about 

 their chemical role, but about their mode of formation. We have mentioned 

 three different ways in which they may be formed. First, indirectly by internal 

 intercombination transition from higher excited states. I am not going to dis- 

 cuss this. Second, by recombination of positive ions and electrons. I am not 

 going to discuss this either. Third, directly. This I shall discuss. What I am 

 going to say is tentative and crude; it came as a surprise to me and perhaps 

 will surprise some of you. It concerns the over-all number of triplet states pro- 

 duced by ionizing radiation, including primary, secondary, etc. processes. We 

 have often said what Dr. Fano said a little while ago, that quite a few triplets 

 are produced directly by the very slow secondary, tertiary, etc. electrons, be- 

 cause when these move slowly enough they can effect singlet- triplet transitions. 

 This, I think, is the general belief, but I don't think that it is true. Mr. Miller 

 and I at Purdue have started a rather formidable project of trying to understand 

 the over-all effects of high-energy radiation in helium gas, by a complicated 

 type of what Dr. Pollard calls "bookkeeping". This work is still in its prelimi- 

 nary stages. But when Jesse found that the metastables in helium could cause 

 increased ionization, we at least had something to aim at besides the value of W, 

 namely, the total number of metastables compared to the total number of ions. 

 We tried to estimate this and it turned out not to be very hard to do, at least 

 crudely. The surprising thing is that most of these metastables appear to be 

 singlets, not triplets. There is, in the case of helium, the possibility of twp dif- 

 ferent types of metastables, singlet metastables and triplet metastables, and 



