The Caesium-Oxygen-Silver Photoelectric Cell 



An Investigation of the Relations in a Composite 

 Photoelectric Surface 



By C. H. PRESCOTT, Jr., and M. J. KELLY 



Technique is described permitting the formation of csesium-oxygen-silver 

 photoelectric cells under controlled conditions. It is shown that the 

 essential conditions are a quantitative control of the degree of oxidation of 

 the silver cathode base and the amount of caesium generated together with 

 a regulation of the amount of chemical interaction by a control of the 

 time and temperature of the heat treatment. 



Variations in sensitivity to integral light at 2,710° K. color temperature 

 are shown as a function of the initial amounts of oxygen and caesium and 

 the time of heat treatment. 



Small amounts of oxygen were permitted to react with the standard 

 cathode surface. The sensitivity of the cathode fell but recovered due to 

 the diffusion of free caesium to the surface from the underlying material. 

 The effects are shown in relation to the integral sensitivity and the spectral 

 response from 6,000 A. to 10,000 A. 



The effects of depositing minute amounts of free caesium upon the 

 standard cathode surface are also shown in relation to the spectral response. 



The active surface of the cathode appears to be a film of free caesium of 

 atomic dimensions adsorbed upon a matrix of caesium oxide and silver con- 

 taining free caesium and a small amount of silver oxide. The spectral 

 characteristics of the photoelectric response appear to depend largely 

 upon the thickness of the surface film of free caesium. This film thickness 

 is determined by the caesium concentration in the underlying matrix and 

 is maintained by a diffusion equilibrium. 



Introduction 



EARLY studies of the photoelectric effect were made on pure 

 metals, eliminating, in so far as possible, the effects of absorbed 

 gases. But since the alkali metals alone respond appreciably to 

 visible light, and these only to light at the blue end of the spectrum, 

 the development of photoelectric cells of greater response to ordinary 

 light sources has led to the study of thin films of the alkali metals 

 and of various composite surfaces. The enhanced photoelectric 

 activity of the thin films of the alkali metals was first brought out by 

 Ives ^ who also noted that the maximum response and the greatest 

 extension of sensitivity toward the red end of the spectrum were 

 obtained when the film thickness was of the order of one molecular 

 diameter. Later work ^ has shown that the maximum excursion of 

 the photoelectric threshold of an alkali metal film on a metallic base 



1 H. E. Ives, Astrophys. J., 60, 4 (1924). 

 " Ives and Olpin, Fhys. Rev., 34, 117 (1929). 



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