USE OP FIELD EMISSION ELECTRON MICROSCOPE 909 



(5) When Ba is deposited on clean W, the average work function ^ 

 decreases from about 4.4 volts to about 2.1 volts when an optimum amount 

 is reached at somewhere near a monomolecular layer. Further deposition 

 increases (p to that of bulk Ba for which (p is 2.5 volts. For convenience we 

 define the average coverage, 6, as the Ba concentration divided by the con- 

 centration when (^ is a minimum. 



(6) For from to 1.0, the emission comes largely from aggregates or 

 clusters of Ba, approximately circular in shape with diameters ranging 

 from 40 to 200 A and a median diameter of about 100 A. Between 600 and 

 900°K these clusters are in violent agitation with the centers of a cluster 

 appearing to shift about half a diameter. Sometimes one cluster may dis- 

 appear and another one near by appear. We propose that this means that 

 the Ba forms two phases on the tungsten surface: a condensed phase of 

 clusters and a gaseous phase of individual Ba atoms. We propose that the 

 centers of these clusters are irregularities on the tungsten surface where 

 small atomic planes or facets meet to form a valley. Even a clean tungsten 

 surface shows evidence of such irregularities whose distribution in numbers 

 and sizes is about the same as for Ba on W but in which the variation in 

 emission density is much less pronounced. 



(7) For ^ > 1.0, the emission comes mostly from larger aggregates which 

 range in size from 200 to 600 A or more. They produce spots which are 

 intensely bright and are in continuous agitation of flicker even at room tem- 

 perature. We associate these larger bright spots with crystallites because 

 we believe them to be caused by Ba crystals which grow out normal to the 

 tungsten surface and thus produce extra large local fields and hence en- 

 hanced local emission. These crystallites disappear, presumably due to 

 migration or evaporation, at temperatures from 400 to 600°K. 



(8) For ^ < 1.0 and r between 600 and 1000°K, the chief effects are due 

 to migration of Ba from one region to another. From 600 to 700°K this 

 migration is restricted to the 211 planes and adjoining regions in the 111 

 zones; the regions near 100 do not yet show migration. In any region migra- 

 tion starts when the Ba clusters show noticeable agitation. At 800°K cluster 

 agitation and migration occur in all regions and Ba atoms migrate from one 

 side of the point to the other side for a distance of 3000 A in about 5 min- 

 utes. At 900°K the migration rate is more rapid. 



(9) For ^ < 1.0 and r between 1050 and 1600°K the chief effect is that of 

 evaporation. This is deduced from the fact that, as the temperature is in- 

 creased progressively in about 100° steps and maintained at each T for 

 about 5 min., the voltage or field required to obtain an emission of say 10 

 microamps becomes progressively higher, presumably because 6 decreases 

 and (p increases. At any one temperature, the rate of evaporation is at first 

 quite rapid but decreases as d decreases. After five minutes the rate is much 



