The Use of the Field Emission Electron Microscope in Adsorp- 

 tion Studies of W on W and Ba on W 



By J. A. BECKER 



The chief conclusions from these studies are given in the Introduction. Table 

 II summarizes the adsorption properties oi W on W and Ba on W. These 

 properties vary with the crystal plane and are given for five planes. The extent 

 to which these planes develop depends on T and the applied field, F. The tem- 

 perature at which W atoms migrate on W at detectable rates depends on the 

 plane and on F, and varies from 800 to 1200°K. 



The adsorption properties of Ba on W are quite different for the first layer 

 than they are for subsequent layers. In the first layer for which 6 < 1, Ba forms 

 two phases: a condensed phase in which the Ba forms clusters or islands having 

 a median diameter of 100 X 10~* cm, and a dispersed phase consisting of indi- 

 vidual atoms. The temperature at which Ba migrates at detectable rates varies 

 from 370 to 800°K from the 110 to the 100 plane. The evaporation rate depends 

 on d. At d near 1.0 it is detectable at 1050°K. At 1600°K practically all the Ba 

 is evaporated. 



For more than one layer of Ba on W, the Ba forms crystallites which grow 

 outward from the W surface even at room T. Their median diameter is about 

 400 X 10-8 cm and they disappear between 600 and 800°K. 



Introduction and Conclusions 



T? W. MULLER,^ in 1936, described a tube in which the field emission 

 -^— ' electrons from a very sharp tungsten point were made to impinge on a 

 fluorescent screen and there portray a magnified image of the variation in 

 emission density from different regions on the point. He showed that magni- 

 fications approaching a million fold could be obtained. In subsequent papers^ 

 he showed how such a tube can yield direct and striking information on the 

 surface structure and on the effects of adsorbed films. Jenkins, ^ in 1943, 

 summarized the progress to that date and showed that fields of the order of 

 10^ volts/cm produced pronounced changes in the surface configuration. 

 More recently F. Ashworth^ has reviewed the field emission from clean metal- 

 lic surfaces. 



In Fig. 2, (a) and (b) are two examples of photographs of the screen when 

 field emission electrons are drawn from a single crystal of tungsten. The 

 bright and dark regions are caused by variations in the intensity of elec- 

 tron emission from different regions of the tungsten surface. From such 

 photographs it is possible to deduce how the electron work function varies 

 for different crystallographic planes, how adsorbed atoms change this work 

 function, and how the surface deviates from a smooth hemisphere when the 

 tungsten is subjected to a range of temperatures and fields. 



It is quite apparent that this new and powerful tool will reveal, on an 



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