54 PHENOMENA, ATOMS, AND MOLECULES 



caesium on the surface. As a matter of fact, the rate of evaporation of the 

 adsorbed caesium from the surface at a thousand degrees is about the 

 same as that from a surface of metalhc caesium at room temperature 

 (300°K). Thus from Trouton's Rule we should expect the heat of 

 evaporation of caesium adatoms to be about three times as great as the 

 heat of evaporation of caesium itself. 



Such large forces between the adsorbed caesium atoms and the underly- 

 ing tungsten surface are to be expected as a result of the fact that the 

 caesium tends to lose its electron when close to the tungsten surface. Thus 

 the positively charged caesium ion induces into the conducting tungsten 

 surface a negative charge, which exerts a force of attraction on the ion 

 (the «image force») equal to eV4x^, where x is the distance of the ion from 

 the surface. This image force is of ample magnitude to account for the 

 strong forces holding caesium on tungsten. 



If a thoriated tungsten filament is used, and this is given the special 

 heat treatment required to bring a complete monatomic film of thorium to 

 the surface, the heat of evaporation of electrons falls to about 3 volts, 

 which is 0.9 of a volt lower than the ionizing potential of caesium. In 

 accord with this fact the experiments show that caesium atoms are not 

 converted into ions by an activated thoriated filament, nor is there any 

 observable tendency for the caesium to become adsorbed on the filament. 



If, with a pure tungsten filament in presence of caesium vapor, the 

 temperature is maintained low enough to permit an adsorbed film of 

 caesium to be formed, the positively charged adatoms, which cause a 

 positive contact potential against the pure tungsten surface, have the effect 

 of lowering the heat of evaporation of the electrons. When such an 

 amount of caesium is present on the surface that the heat of evaporation 

 is materially below 3.9 volts, the tendency of the filament to rob incoming 

 caesium atoms of their electrons and thus allow them to escape as ions is 

 lost. This means that Vp, the rate of evaporation of ions, becomes very 

 small as 0, the fraction of the surface covered by caesium atoms, increases. 



At temperatures of about 700°K, in presence of caesium vapor saturated 

 at room temperature (pressure about io~^ atmospheres), increases to 

 approximately 0.7 and the heat of evaporation has then been lowered to 

 such a point that the electron emission is 10^" times greater than that of a 

 clean tungsten filament at the same temperature. 



Within the last three years, Dr. J. B. Taylor and I have made a de- 

 tailed study (28) (30) of the rates of evaporation of atoms, ions and 

 electrons from these caesium films on tungsten as functions of and T. 

 In order to measure 0, a method was devised for determining 0, the num- 

 ber of adsorbed atoms per square cm. of tungsten surface. Two methods 

 were found, When is less than 0.08, a sudden heating or flashing of the 



