352 BELL SYSTEM TECHNICAL JOURNAL 



equivalent to four layers of casium upon a smooth cathode, would be 

 sufficient to oxidize either the monatomic film of caesium on the bulb, 

 or one atomic layer upon the cathode. We have therefore supposed 

 that about half an atomic layer upon the surface of the cathode was 

 actually oxidized. With the other three cells we have subtracted 

 four "layers" for the bulb and divided the remainder by four to 

 obtain a rough estimate of the actual number of atomic layers of free 

 caesium upon the cathode surface which were oxidized. These values 

 are shown in the third row of Table 2. 



Each cell was first baked for 30 minutes at 75° C. to insure that the 

 surface was in substantial equilibrium. No integral sensitivity 

 changed by more than 10 per cent, and changes in the distribution of 

 spectral response were inappreciable. These results are in agreement 

 with our general experience that these cathodes are stable at 75* C. 



All four cells were radically affected by the introduction of oxygen, 

 but none were made entirely inactive. All recovered somewhat at 

 room temperature but the rate of recovery slowed down rapidly. 

 After the recovery had flattened out at room temperature the cells 

 were baked at 75° C. The process of recovery was much accelerated 

 so that cell A recovered most of its initial sensitivity, but cells B, C 

 and D did not recover a high sensitivity until after a short heat 

 treatment in an oven at 200° C. Cell A losing half a layer of caesium 

 upon the introduction of oxygen fell to 47 per cent of its initial sensi- 

 tivity. It recovered to 59 per cent after 48 hours at room temperature, 

 and to 89 per cent after 3 hours at 75° C. Cell B losing one layer 

 fell to 16.5 per cent, recovered to 27 per cent after 68 hours at room 

 temperature, to 60 per cent after 14 hours at 75° C, and to 83 per cent 

 after 15 minutes total at 200° C. Cell C losing six layers fell to 

 0.95 per cent, recovered to 3.5 per cent after 12 hours at 75° C, and 

 to 89 per cent after 5 minutes at 200° C. Cell D losing eleven layers 

 fell to 0.3 per cent, recovered to 1.4 per cent after 7 hours at room 

 temperature, to 2.8 per cent after 6 hours at 75° C, and to 68 per cent 

 after 20 minutes total at 200° C. 



The heat treatment at 200° C. was kept short as reaction between 

 free caesium and silver oxide and evaporation of caesium oxide both 

 occur at this temperature and in time would destroy even a normal 

 cathode surface. In the five-minute heat treatments the maximum 

 cathode temperature was doubtless considerably less than 200° C. 

 and the increased effects of the ten-minute treatments which followed 

 are probably due more to the enhanced cathode temperature than to 

 the longer time. The observed recovery must be due to the migration 

 of free caesium already present as there is no chemical reaction possible 



