908 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



almost atomic scale, the nature of adsorption phenomena which are basic 

 to thermionic, photoelectric and secondary electron emission, to catalysis 

 and also to biological processes. Unfortunately, in most of the early work the 

 residual gas pressure in the tube was such that the surface was contaminated 

 in a few minutes and hence the results were probably affected by this con- 

 tamination. In the present investigation the vacuum conditions were im- 

 proved to such an extent that the residual gas produced only barely detec- 

 table effects after about one week. Under such conditions the following 

 observations and conclusions Have been made: 



(1) When a sharp point of a single crystal of tungsten is held at 2400°K 

 until a steady state is reached, most of the surface is approximately hemis- 

 pherical or more precisely paraboloidal. About 20% of the surface consists 

 of three atomic planes which in decreasing order of size are 110, 211, and 100 

 planes. This is also the order of increasing intensity of field emission. The 

 greatest intensity of emission is from the 611 direction and other directions 

 surrounding the 100 direction. The next greatest intensity comes from the 

 111 direction and neighboring regions. 



(2) For temperatures > 2400°K the area of the 110, 211, and 100 planes 

 decreases ; between 2400 and 1050°K the 211 and 100 planes increase steadily 

 in size ; below 1050°K the rate of change of area is so slow that no changes 

 are observed in one hour. These changes are ascribed to migration of W 

 atoms on W. 



(3) From 1050 to 1200°K, W atoms migrate most easily in the 111 direc- 

 tion on the 211 plane. In this direction the atoms in the outermost layer 

 contact their nearest neighbors but the rows of atoms are separated by 

 1.635 atom diameters. The migrated W atoms are deposited on the hemis- 

 pherical surface adjoining the 211 plane and form a crescent shaped mound 

 resulting in an abnormally high field and enhanced emission. In the region 

 between the 211 and 110 planes, W atoms are also mobile in the 111 direc- 

 tion and form a series of step-like planes. In other regions the W atoms 

 show no large scale migration. Above 1200°K, W atoms are mobile every- 

 where. 



(4) When fields of the order of 40 million volts/cm are applied to the sur- 

 face the rate of change of the surface configuration is greatly increased and 

 migration of W atoms can be observed in one hour on the 211 planes and 

 near-by regions at 800^K. These changes are the same for electron accelerat- 

 ing and electron retarding fields. The rate of change increases rapidly with 

 the strength of the field, perhaps as the square or cube of the field. At T = 

 1400°K and for fields of 40 X W volts/cm applied for hours, over half of 

 the surface consists of planes: the 211 planes almost meet the 110 planes, 

 and HI and 310 planes develop. Subsequent glowing without an applied 

 field undoes the effects produced by the field. 



