EVAPORATION OF ATOMS 375 



Let us now assume provisionally that t is the same for all exposed 

 adsorbed atoms in the 2nd (or 3rd) layer on tungsten as for atoms in a 

 surface of metallic caesium at the same temperature. We thus take x to be 

 independent of the surface concentration of atoms although for the ist 

 adsorbed layer on tungsten the strong repulsive dipole forces between 

 adatoms cause t to decrease as 61 increases. Since Cs atoms on a layer 

 already covered by caesium probably have very small dipole moments, and 

 any small moment that does exist may be compensated for by attractive 

 forces, it seems reasonable to make this simplifying assumption. 



We may then put 



and under steady conditions in which v = ^i we then have from Eq. (49) 



logio(MM)= 27.37 -3840/r. ^^SO 



Here O2 represents the fraction of the available part (^1^) of the first layer 

 which is covered by the adatoms in the second layer. The total number of 

 atoms in the second layer per unit area of true tungsten surface is thus 



When [I increases to the value \u corresponding to saturated vapor at 

 the temperature of the filament, Q2 must rise to unity. Thus by Eq. (51) 

 we have an equivalent definition of ^2 



62 = 11/ 112. ^52) 



In experiments on transients \x, or v must have values which give reason- 

 able time intervals for coating or depleting the surface. With |x = 10^^ 

 the coating time to ^ = i is one hour and with 10^^ it is 0.4 second. Let us 

 therefore choose these values of \i and calculate by Eq. (6) for various 

 values of 6n the corresponding temperatures. These data are given under T 

 in Table V. 



With these values of T, putting v = |x we calculate by Eq. (51) the 

 values of 62 given in Table V. The values of T in the lowest line are those 

 obtained from Eq. (51) by putting 62= i; these are the temperatures at 

 which liquid caesium (polyatomic layers) would condense on the filament. 



Examination of these data shows that Oi'^Oo, the number of caesium 

 atoms per unit area in the second layer needed to give an evaporation rate 

 equal to |j. is extremely small until 6\ reaches values of about 0.96. For still 

 higher values of 6^, 62 increases rapidly. When 62 becomes comparable with 

 unit, we must take into account the adsorption in the third and higher 

 layers. 



