^16 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



ing; the lower part describes the conditions under which the photograph 

 was taken. For example, for Fig. 2 (a) the W point was heated to 2400°K 

 with zero applied voltage for 30 sec. The photograph was taken with the 

 point at room temperature, assumed to be 300°K, with +9000 volts applied 

 to the anode, while a field emission current of 40 microamps was drawn 

 from the point, and for an exposure of J sec. 



From Fig. 2(a) it is easily seen that for "normal clean W'^ which we define 

 as W glowed at 2400°K, the 110, 211, and 100 regions emit poorly; the 111 

 region emits moderately; the greatest emission density comes from a rather 

 broad band surrounding the 100 region. The center of this band makes an 

 angle of about 20° with the 100 direction. The 611 plane lies in the central 

 part of this band. In Jenkins' Report^ it is shown that the 110, 211, and 100 

 dark regions are planes and that the extent of these planes can be increased 

 by applying high positive fields while the point is at temperatures near 

 1200°K. 



Figure 2(b) shows a photograph for a point which has been treated in 

 this manner; Ba was then evaporated onto the W and the W loop was 

 heated until the Ba migrated over the surface. Figure 3(a) shows the emis- 

 sion from migrated Ba on normal clean W. In Figs. 2(b) and 3(a) the amount 

 of Ba is rather small, about .10 monolayer. For Ba on tungsten the emission 

 comes from small "circular" regions with an average diameter of 100 A. 

 We interpret these to be small regions in which the Ba atoms cluster together, 

 thus reducing the work function more than in neighboring regions, and hence 

 we call such regions clusters. 



A careful inspection of the negatives for clean W show that, in regions 

 other than the 110, 211, and 100 planes, the emission shows a granular 

 structure with small regions of the order of 100 A diameter surrounded by 

 slightly darker regions. We believe this to be due to submicroscopic facets 

 on the paraboloidal surfaces; these facets form small hills or plateaus and 

 valleys. On the small hills the local field is slightly greater than in the valleys 

 and hence the emission from clean W is slightly greater than from the val- 

 leys. On the other hand we believe that the Ba atoms are held more firmly 

 in the valleys or troughs where they can contact more W atoms, thus 

 accounting for the Ba clusters discussed in the preceding paragraph. 



Figure 2(b) also shows a series of large elliptical rings with their center 

 in the 1 10 plane and their major axis in the 100 zone. The evidence for these 

 is especially pronounced in the HI zones. The separation between them is 

 about 170 A. This suggests that the 110 plane and the region surrounding 

 it consists of a series of plateaus of 110 planes elliptical in shape. 



