158 PENETRATION PHENOMENA IN LIQUID WATER 



cause atomic hydrogen is not accessible to stopping-power studies, and 

 the binding of hydrogen in different molecules is of a particularly uni- 

 form character. One would anticipate only minor deviations (ordinarily 

 smaller than 1 per cent) in the contribution of hydrogen to the stopping 

 power for fast particles of various gaseous compounds, the mean value 

 probably being close to the value for ^H2. Nevertheless the analysis 

 for H2, the simplest of all molecules, is of interest as a guide to the 

 understanding of more complex molecules. 



For such molecules the alterations in fn, Zn values arising from molec- 

 ular binding will be vastly more complex, and there is hardly hope that 

 they will yield to detailed theoretical analysis. Of course, there may be 

 discovered some method of deducing the effect of chemical binding on 

 the stopping power in a general way, without elaborate analysis of the 

 /„, e„ values. Some promise is also offered by the possibility of determin- 

 ing and analyzing the ultraviolet absorption spectra (including continua). 



Only transitions involving a valence electron will be strongly affected 

 by molecular binding. Thus deviations are likely to be suppressed for 

 all but the lightest elements, notably C, N, O, and F. Because the 

 character of the valence bonding in molecules containing these atoms 

 can vary most markedly, significant deviations from the Bragg rule 

 should be anticipated. Double bonds, triple bonds, and resonating- 

 group structures (such as benzene) seem especiallj^ suggestive in this 

 respect. Because of the inert character of the inner electrons, however, 

 deviations exceeding about 5 per cent will be rare. Molecules, such as 

 NO and NO2, having an odd electron, may present unusual features. 



V. Stopping Power of a Solid or Liquid 



The Bragg rule of additivity of stopping powers is usually presumed 

 to be applicable to liquids and solids as well as to polyatomic gases. One 

 would anticipate, however, that the rule should in most circumstances 

 be even more in error for condensed media, in so far as the contribution 

 to the stopping power by outer electrons is concerned, because in ad- 

 dition to effects of chemical binding there may enter effects of inter- 

 atomic or intermolecular interactions involving several or many units 

 of the medium. 



Such "collective" effects have been investigated only * for the special 

 and important case of metallic conductors. Here the valence electrons 



* In this paper we exclude problems of relativistic velocities, at which there occurs 

 a decrease in stopping power, arising from screening of the field of the moving parti- 

 cle by the excited and ionized atoms in its wake, that may be very important for 

 condensed substances. This effect, which has attracted much attention in recent 



