C^SIUM-OXYCEN-SILVER PHOTOELECTRIC CELL 353 



under these conditions by which free caesium could be obtained from 

 the caesium oxide present. 



This recovery of sensitivity has every appearance of accompanying 

 a diffusion process by which caesium in the matrix is brought to the 

 surface of the cathode. Initially the concentration is uniform through- 

 out the matrix. As soon as the oxygen is released it oxidizes the 

 caesium on and near the surface until the oxygen is exhausted. The 

 caesium oxide formed is identical with that already present. The 

 resulting condition will be a matrix with a very sharp gradient from 

 outer surface inward in caesium concentration. This will cause a 

 rapid initial rate of outward diffusion. But as soon as diffusion occurs 

 the concentration gradient is decreased and the rate of transfer is less 

 rapid since the caesium is transported over a constantly greater 

 distance under a decreasing diffusion pressure. As the temperature 

 is raised the mobility of the caesium is greatly increased but the 

 essential nature of the process is the same. 



It does not appear that any conclusion can be drawn from these 

 data as to the thickness of the surface film of caesium. Even in cell D 

 with oxygen equivalent to eleven atomic layers of caesium there was 

 still a finite residual activity immediately after the oxygen reacted, 

 which was not far different in value from that obtained with the six 

 equivalent layers in cell C. It seems probable that the matrix is 

 sufficiently spongy that there is a rapid diffusion of oxygen into it 

 and immediate interaction of oxygen with the absorbed caesium, 

 preventing a complete clean-up of all caesium on the surface with the 

 amounts of oxygen used. Cell D is also noteworthy in that the oxygen 

 used was equivalent to 7 per cent of the total caesium found on such 

 a cathode. And after the oxidation of this amount of free caesium 

 it was still possible to develop from this surface a high sensitivity 

 equal to two thirds that of the initial state. So the initial amount 

 of free caesium must be still greater than 7 per cent of the total caesium 

 on the surface. 



But it is not necessary to suppose a much greater amount since it is 

 known that monomolecular films have a great tendency to form. 

 This may resolve the difficulty in understanding why, after the 

 equilibrium is reestablished, so large a change in volume concentration 

 involves so slight a change in the surface condition. While the 

 thickness of a surface film in equilibrium with a gas or vapor is a 

 definite function of the pressure, the film thickness changes much less 

 rapidly than the pressure when the film is in the neighborhood of one 

 molecule thick. The same type of functional relationship may also 

 obtain when the surface concentration is determined as a function 



