Energy Distribution of Photoelectrons as a Function of the Thickness of a Potassium Film

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Abstract
Current-voltage curves were taken for photoelectrons ejected from thin potassium films of various thicknesses. The films were formed on a silvered glass surface at room temperature by means of a molecular beam. The condensing surface was a small sphere placed at the center of a large collecting sphere. The thickness of the potassium film was computed from the geometry of the photocell, the temperature and time of heating of the potassium reservoir, and data on the vapor pressure of potassium. The small inner sphere was connected to an electrometer which measured the photocurrent. The contact potential between the film and the silvered surface of the large sphere was determined from the values of the thresholds of the two surfaces. The form of the curves near the maximum retarding potential was approximately the same for each film thickness. The curves were analyzed by DuBridge's method and were in good agreement with the theory. For films less than 3 molecular layers corrections had to be made for a constant decrease in photocurrent after the formation of the film. However, for films of greater thickness the photocurrent was quite constant. The photocurrent for films less than 3 molecular layers failed to saturate for accelerating potentials up to 20 volts. As the film thickness increased the photocurrent began to saturate at lower voltages.