DISCUSSION OF MORRISON'S AND FANO'S PAPERS 19 



DISCUSSION OF MORRISON'S AND FANO'S PAPERS 



ZiRKLE : 



Is the probability of capture of electrons by the oxygen atom markedly greater 

 than the probability of capture of the electron by water? 



Fano : 

 The oxygen atom in the water molecule is the capturing agent. 



Latarjet : 



Fano stated that atoms with low ionization potential, such as hthium and 

 cesium, are more easily excited, rather than ionized, as compared to atoms with 

 a higher potential. Would this remark be of general value and be applicable to 

 light atoms in the liquid or solid state, that is to living tissues? 



Fano : 



Yes. It is pointed out, however, that the elements in biological systems are 

 all in the same bracket of ionization potential, except for the few metallic atoms. 



Latarjet : 



The ionization potential in this range varies from 9 to 20 volts. 



Fang : 



Only xenon and hehum have ionization potentials of about 9 volts. If negative 

 ions, the probabihty of excitation is greater if excitation is to occur at all. 



Burton: 



I wish to comment on the previous two questions (by Zirkle and Latarjet) 

 and on the rephes. 



It is undoubtedly true that the O2 molecule itself can capture thermal elec- 

 trons to give negative ions and that this is an important process in the gaseous 

 state. Threshold energy for the capture of electrons by water in the gaseous 

 state makes the cross section of this process so high that it does not compete 

 effectively with capture by positive ions. In the liquid, however, solvation of 

 the 0H~ contributes much energy. The potential-energy curve is so displaced 

 that H2O now captures an electron in a dissociative process to yield 0H~ (aq) 

 without threshold. The cross section for such a process is, of course, lower than 

 the cross section for capture by positive ions, but the concentration of water 

 molecules is so overwhelmingly higher than the concentration of positive-ion 

 species present that the process e + H20- aq H + 0H~- aq becomes very 

 important. 



In regard to the question of the relative probabihty of formation of two 

 species of positive ions with distinctly different ionization potentials, it is im- 

 portant to remember that, although both will be formed initially, the ion of 

 higher ionization potential transfers its charge to the other species. There are 



