130 



The Electron Microscope 



infinity for ^ ^ 0, while 1(0) for hydrogen, or any other neutral 

 atom is large but finite. Consequently the total scattering cross 

 section of an ion is infinite, because the long-range Coulomb 

 force aflfects electrons appreciably at any distance. This is the 

 explanation of Hillier's paradoxial result, ""^'^ that an atom, for 

 which he assumed Rutherford's Law, ought to produce the best 

 contrast at zero aperture. 



KSWmd 

 5 



1 



H '^ATOM 



(motts^massey) 



\ V' 10,000 VOLTS. 



Fig. 46. Scattering by hydrogen atom 



The total scattering cross section of a neutral atom on the 

 other hand is finite. In the instance of a hydrogen atom and 10 

 kev electrons it is 4.26 X 10~^^ cm-, corresponding to a disk of. 

 0.23 A diameter. Applying the considerations in chapter 8, we 

 can say at once that a single hydrogen atom can become visible, 

 i.e., can produce a contrast of at least 10 per cent in an electron 

 microscope of the transmission type only if the resolution limit 



. 0.23 



IS 



0.3 



0.77 A or better. This would mean improving the 



