APPLICATION OF ELECTRON DIFFRACTION 



601 



in intensity of the "Type-4" beams from one curve to another. 

 In our original pubhcation we pointed out a possible interpretation of 

 these intensity differences. The data concerning these beams have 

 now been more carefully studied, and it has turned out that this 

 suggested interpretation gives quantitative results in agreement with 

 the observations. 



What we suggested was that the changes in the intensities of the 

 "Type-4" beams were due to interference between the beams diffracted 

 from the layer of gas atoms and the beams arising from the underlying 



SURFACE NICKEL ATOM 



) ADSORBED GAS ATOK/( 



CIRCLES INDICATE AREAS 

 PARTIALLY SHIELDED BY 

 GAS ATOMS 



Fig. 10 — The arrangement of gas atoms which gives rise to the "Type-4" diffrac- 

 tion beams. The designation of crystal azimuths is the one upon which the calcula- 

 tions of Table I are based. 



nickel. That the nickel crystal can produce diffraction beams capable 

 of interfering with the beams from the doubly-spaced layer of gas 

 atoms arises from the fact that the gas atoms divide the surface nickel 

 atoms into two classes. Three-fourths of the surface nickel atoms are 

 adjacent to gas atoms and are presumably partially shielded by these 

 atoms from the incident electron waves (Fig. 10). The other fourth of 

 the nickel atoms on the surface, which are not so shielded, are arranged 

 in a structure similar to that of the gas atoms and having the same 

 scale factor. Thus it is that the surface layer of nickel atoms can give 



