602 BELL SYSTEM TECHNICAL JOURNAL 



rise to differential diffraction beams capable of interfering with the 

 "plane grating" beams from the gas atoms. 



Believing that the intensity differences of the beams in Figs. 6-8 

 arise in this manner, one can hope to determine from the intensities of 

 these beams the space relation existing between the layer of gas 

 atoms and the surface layer of nickel atoms. In carrying out the plan 

 of making this determination one naturally assumes a certain relative 

 spacing, and upon this assumption calculates the wave-lengths (and 

 angles) at which the diffraction beams from the gas layer are in phase 

 with the differential beams from the surface nickel layer.^" These 

 wave-lengths are then compared with the wave-lengths in Figs. 6-8 

 at which the "Type-4" beams are found to be strong. 



Calculations of this kind have been carried out, with the gas atoms 

 placed as shown in Fig. 10, for each of a series of assumed separations 

 between the plane of the gas atoms and the plane of the surface nickel 

 atoms. In comparing the results of these calculations with the 

 observations of Figs. 6-8, the azimuths are to be taken either as shown 

 in Fig. 10 or with the (HI) and (100) azimuths interchanged. As far 

 as the first layer of nickel atoms is concerned the (HI) and (100) 

 azimuths are identical. When one must distinguish between these 

 azimuths (as in Fig. 10), this distinction is equivalent to a certain 

 definite location of the gas atoms relative to the deeper layers of nickel 

 atoms. It turns out that, when the centers of the gas, atoms are as- 

 sumed to lie in a plane separated by 3.0 A. from the plane of the 

 centers of the surface nickel atoms and when the azimuths are chosen 

 as shown in Fig. 10, the calculated wave-lengths of the intensity 

 maxima of the "Type-4" beams agree pretty well with the wave- 

 lengths at which the beams are observed to become strong. These 

 calculated wave-lengths of the intensity maxima are given in Table I. 



TABLE I 



Calculated Wave-lengths of the Intensity Maxima of "Type-4" Beams 

 Assuming the Azimuth Designations of Fig. 10 and a Separ.\tion 



OF 3.0 A. BETWEEN THE PlANE OF THE GaS AtOMS AND 



THE Plane of the Surface Nickel Atoms 



^^ One does not, of course, obtain in this simple manner the intensities of the 

 diffraction beams, but one does obtain the positions and wave-lengths at which these 

 beams reach intensity maxima. 



