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BELL SYSTEM TECHNICAL JOURNAL 



Fig. 1 consists of two electron scattering curves showing a (422) 

 reflection which occurs at an electron wave-length of 1.52 A. The 

 curve on the left was taken before the crystal had been heated after the 

 sealing of the bulb containing it from the pumps. In the curve on the 

 right is shown this same diffraction beam soon after the crystal was 

 heated. From the reasoning given above we know that at the time 

 the left-hand curve was taken the crystal surface was covered by many 

 layers of adsorbed gas, and that when the right-hand curve was taken 

 the surface of the crystal was comparatively clean. 



Fig. 2 is similar to Fig. 1. It shows the increase in intensity of a 

 (622) reflection when the crystal surface was cleaned by heating. 

 After such a cleaning the surface became gradually covered again by 



{622} REFLECTION 



CONTAMINATED SURFACE 



{622} REFLECTION 



CLEAN SURFACE 



Fig. 2 — The effect of the removal of surface gas upon a (622) diffraction beam 



adsorbed gas. The intense diffraction beams shown at the right of 

 Figs. 1 and 2 became weaker, finally reaching the intensities shown by 

 the left-hand curves. The time required for this change to occur 

 varied from less than an hour to many weeks, depending upon the 

 vacuum condition of the bulb. 



In Fig. 3 is shown the effect of cleaning the crystal surface upon the 

 intensity of a (331) reflection, and in Fig. 4 the effect upon a (551) 

 reflection. The curves either side of the centers in these figures show 

 how selective are the occurrences of these diffraction beams. The 

 disappearance of the diffraction beams as the wave-length is changed 

 from the critical value of 1.67 A. for the (331) reflection and .912 A. 

 for the (551) reflection respectively is much less sudden than the 



