POLYATOMIC MOLECULES 77 



with an electron will give an average yield of between two and three H 

 atoms per Hs"^. This is the principal neutralization reaction. Com- 

 bining it with the previous arguments, one obtains a total H-atom yield 

 of about 6 per ion pair produced. The remaining reactions to be con- 

 sidered are 



H + H = H2 

 and 



H + H2(p) = H2(o) + H 



The first reaction requires a third body and occurs essentially only on 

 the walls of the vessel. The second reaction is very rapid and effective 

 in causing the conversion. Calculations based upon the known diffusion 

 constant of H atoms in H2 and the rate of the last reaction above give 

 results in good agreement with experiment. 



The synthesis and decomposition of hydrogen bromide by alpha 

 particles in hydrogen-bromine-hydrogen bromide mixtures were treated 

 in similar fashion (17). The primary ionization processes were obtained 

 from mass spectral data. Of the large number of secondary reactions 

 possible, many were immediately discarded as too slow to be important, 

 and an analysis of the kinetics of the system was made in terms of the 

 remaining reactions. It was found that the data could be satisfactorily 

 explained with the assignment of reasonable values to the few unknown 

 rate constants. A similar treatment of the alpha-particle-induced re- 

 actions in carbon monoxide-oxygen-carbon dioxide systems has been 

 made by Hirschf elder and Taylor (18). 



Polyatomic Molecules 



One is naturally inclined to attempt to extrapolate from the reasonably 

 well understood actions of diatomic molecules to polyatomic molecules 

 (19). There are many points of similarity. Each ion formed from a 

 polyatomic molecule has a well-defined appearance potential, and the 

 ionization-efficiency curve for the production of an ion is of the same 

 general shape as similar curves for diatomic molecules. With small 

 polyatomic molecules such as CO2 and CH4, one finds ions formed with 

 appreciable amounts of translational energy. However, when we 

 examine the mass spectra of hydrocarbons the size of propane and 

 larger, we find the spectra show certain distinctive features different 

 from small polyatomic and diatomic molecules. 



The mass peaks for ions obtained from large molecules are almost 

 invariably symmetrical and of a shape determined only by the character- 

 istics of the instrument. This fact, important in permitting the determi- 



