EVAPORATION OF ATOMS 325 



of lattice (body-centered cubic) as tungsten and the lattice constant 

 (6.17A) is almost exactly double that of tungsten (3.15A). Thus if, in a 

 complete nionatomic film of caesium on tungsten, the caesium atoms 



Fig. I. True (S) and apparent (.S^i) filament surface. 



arrange themselves in a surface lattice identical with that of the tungsten 

 surface but with double spacing, there will be one caesium atom for every 

 four tungsten atoms, all the adatoms will be similarly placed with respect 

 to the underlying tungsten atoms, and the adatoms will be at the same 

 distances from one another as in metallic caesium. 



Because the strong forces between the caesium and the tungsten atoms 

 must tend to make the adatoms occupy definite elementary spaces ^ de- 

 termined by the tungsten lattice, we believe we are justified in adopting this 

 4 to I relation as a postulate in our further investigations. We may there- 

 fore put for the true surface concentration Oi of a complete caesium film 

 on tungsten 



01 = 3.563 X 10^^ atoms cm"2. (2) 



Experiments which will be described in Section V and XII have shown 

 that with the particular filament used, the apparent surface concentration 

 0^1 for a complete film of caesium on tungsten is 



Oa.1 = 4.80 X 10^* atoms cm"-. (3) 



The ratio o^i/oi, which is 1.347, is thus eqtial to S/Sa, the ratio of 

 the true to the apparent filament surface. Tonks has calculated ^ that with 

 a random distribution of dodecahedral crystals the minimum value of S/Sa 

 is 1.225, ^ value which supports our postulate. 



We intend to test this postulate in future work by comparing values of 

 Gai obtained with adsorbed films of caesium, rubidium and potassium on a 

 single tungsten surface. If the value Oi as given above is correct, we should 

 expect that Oai will be the same for all alkali metals. 



Let Oa be the apparent surface concentration of adatoms obtained by 



dividing the total number of adatoms by Sa, the apparent surface area 



(nld). Then the true surface concentration a is OaSa/S. It is probable that 



S/Sa may vary somewhat for dififerent filaments according to the aging 



treatment and the fineness and orientation of the crystals. The intrinsic 



properties of the adsorbed film are presumably dependent on a rather than 



®1. Langmuir, Jour. Amcr. Chan. Soc, 38, 2287 ( 1916) ; 40, 1366 (1918). 

 ' L. Tonks, Phys. Rev., 38, 1030 (1931). 



