11 



reasons I think a multiple excitation process is rather unlikely. 



BURTON: All I did was to give an experimental situation that corresponded 

 to the model suggested by Dr. Kasha. For a parallel situation in a condensed 

 system there would have to be some very strict selection rules forbidding the 

 deactivation transition. 



PLATZMAN: May I return the discussion to the subject for this morning, 

 and ask Dr. Tobias if he would care to speak now? 



TOBIAS: Theos Jardin Thompson, working with Segre and Chamberlain, 

 wrote a recent Ph.D. thesis on the effect of chemical structure on stopping pow- 

 er (11). He studied the stopping power of 380 Mev protons with 270 Mev mean 

 energy in various inorganic and organic compounds having different percentages 

 of hydrogen, carbon, oxygen, nitrogen, and chlorine. He concluded that the in- 

 fluence of chemical binding is less than 1 per cent, that is, the elements in the 

 various compounds have the same stopping power within one per cent. He could, 

 however, identify small but measurable deviations from the additivity of stopping 

 powers of elements from compounds. From his work there resulted a number 

 of quite accurate stopping power determinations for hydrogen, which is of special 

 interest, as well as for other elements. 



POLLARD: You don't have the effective I values, do you? 



TOBIAS: Thompson gives the relative molal stopping powers (S) with re- 

 spect to copper and the mean ionization potential, _L Some of his values are 

 listed in table I. 



TABLE I 



RELATIVE STOPPING POWER AND MEAN IONIZATION POTENTIAL 



OF VARIOUS ATOMS 



Element S I 



Hydrogen (molecular) 0. 0472 + 0. 0002 (est. ) 18.2 ev 



Carbon (graphite) 0. 2455 + 0. 0005 (est. ) 70.2 ev 



Nitrogen (molecular) 0. 2837 +_ 0. 0001 (est. ) 76.3 ev 



Oxygen (molecular) 0. 3188 +_ 0. 0003 (est. ) 88.3ev 



Chlorine (from com - 



pounds) 0.6335 + 0.0035 153.7 ev 



FANO: I have this only by word of mouth. The striking thing was that from 

 the value of effective I you could really say how many double bonds there were 

 in a molecule. 



PLATZMAN: That contradicts the statement that the effect of chemical 

 binding is negligible. 



FANO: The experiments are accurate within a very small fraction of one 

 per cent; and that is the striking thing, that one per cent effects could be de- 

 tected. 



