SURFACE CHEMISTRY 53 



Thus we see that the rate of evaporation should increase indefinitely as 

 approaches unity. It is just this effect which limits the amount of material 

 in typical adsorbed films on liquids or solids to that contained in a mono- 

 molecular layer. 



The forces holding adatoms on the surface in characteristic cases of 

 adsorption are usually far greater than those acting between adatoms. If 

 this were not the case, we would be dealing normally with the case in which 

 Vo is less than Vi, so that we would not have adsorbed films but crystal 

 nuclei. The effect of these strong forces originating from the underlying 

 surface is to polarize the adatoms. If these are all of one kind, they thus 

 tend to become similarly oriented dipoles which repel one another with a 

 force varying inversely as the fourth power of the distance between them. 

 There may also be attractive forces of the Van der Waal's type, but these 

 will vary with a much higher power of the distance. The dipole forces will 

 have a range of action far greater than other forces involved. A knowledge 

 of these factors which are probably important in most cases of adsorption 

 can best be acquired by a detailed study of some one example in which 

 quantitative determinations of all the factors are possible. The example 

 that I have chosen for this purpose is that of adsorption of caesium vapor 

 on tungsten, for in this case we can measure with great precision the 

 surface concentration o as well as the rates of evaporation v of atoms, of 

 ions, and of electrons (30). Thus we are able to express these values of 

 Va, Vp and Vg as functions of and T. These measurements enable us not 

 only to determine the forces acting between the adatoms but also to de- 

 termine the electrical properties of the adsorbed films. 



Caesium Films on Tungsten. The ionizing potential of caesium is 3.9 

 volts, which is lower than that of any other chemical element. The heat of 

 evaporation of electrons from tungsten corresponds to 4.6 volts. Thus the 

 energy necessary to detach an electron from a caesium atom is 0.7 volts less 

 than that necessary to pull an electron out of metallic tungsten. It is there- 

 fore not surprising that experiments (31) (32) show that every caesium 

 atom which strikes a tungsten filament at high temperature loses its electron 

 and escapes as a caesium ion. The electric current that thus flows from the 

 tungsten is a quantitative measure of the number of atoms of caesium that 

 strike the filament. Since currents of io~^^ amps, can be measured by an 

 electrometer, it thus becomes possible to detect a pressure of caesium vanor 

 so low that only 100 caesium atoms per second strike the surface of a 

 filament. 



The escape of the caesium ions from the tungsten surface is an evapora- 

 tion phenomenon. If the temperature is below about i,ioo°K, the rate of 

 evaporation of the ions may be so low that it does not keep pace with the 

 rate at which the atoms arrive, so that there is an accumulation of adsorbed 



