I 



An Application of Electron Diffraction to the 

 Study of Gas Adsorption 



By L. H. GERMER i 



Under appropriate experimental conditions, electron scattering by a single 

 crystal of nickel can give rise to diffraction patterns of four quite distinct 

 types. We attribute one of these patterns to the space lattice of the nickel 

 crystal, one to the topmost layer of nickel atoms, one to a monatomic layer 

 of adsorbed gas atoms, and one to a thick layer of gas atoms. From these 

 phenomena some conclusions concerning gas adsorption have been drawn. 

 We have at hand a new and important method of crystal analysis. 



IN the paper by Dr. C. J. Davisson and myself entitled "Diffraction 

 of Electrons by a Crystal of Nickel," ^ we published a variety of 

 information concerning the gaseous contamination of the surface of our 

 diffracting crystal. This information was obtained from a study of 

 the modifications produced by adsorbed gas in the electron diffraction 

 pattern. We have subsequently succeeded in obtaining some addi- 

 tional facts concerning adsorbed gas from a further study of our original 

 data. Along with the presentation of these new facts, I am taking 

 this opportunity to publish in greater detail the data upon which our 

 original conclusions were based. 



In our Physical Review paper we showed that the interaction of a 

 beam of electrons with a single crystal of nickel gives rise to phenomena 

 which, in their most essential characteristics, are similar to the diffrac- 

 tion phenomena which w^ould be observed if the beam of electrons of 

 adjustable speed were replaced by a beam of X-rays of adjustable 

 wave-length. The diffraction patterns produced by electron scattering 

 are, however, substantially more complicated than would be the cor- 

 responding X-ray diffraction pattern. We showed that electron 

 scattering can give rise to diffraction phenomena of four quite distinct 

 types. The diffraction patterns of two of these types arise from the 

 nickel atoms in the crystal lattice, while the diffraction patterns of the 

 other two types have their Origins in the layer of gas adsorbed upon the 

 surface. The relative intensities of the diffraction patterns of these 

 four types are determined by the amount of gaseous contamination on 

 the surface of the crystal, and also by the temperature of the surface. 



The first publication of our discovery of electron diffraction was con- 

 tained in a note in " Nature." ^ At the time of this first paper we had 

 already discovered two of these types of electron diffraction, and had 

 distinguished sharply between them. The first type, which we later 



1 Translation from "Zeitschrift fur Physik," April 12, 1929, pp. 408-421. 



2 C. J. Davisson and L. H. Germer, Phvs. Rev., 30, 705 (1927). 

 » C. J. Davisson and L. H. Germer, Nature, 119, 558 (1927). 



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