Photoelectric Properties of Sodium Films on Aluminum

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Abstract
Previous work has shown that thin films of potassium, rubidium, and caesium deposited on a silver surface give a maximum sensitivity when the thickness of the film is of the order of ten molecular layers or less. As the deposit continues the sensitivity decreases before reaching a constant value. Sodium films were deposited on aluminum, and it was found that the maximum sensitivity was not attained until the thickness of the film had reached a value of eighty molecular layers. The sensitivity was then found to remain constant for greater thicknesses. No detectable photo-current was observed until the film reached a thickness of five molecular layers. Spectral distribution curves were obtained and analyzed according to Fowler's theory. This yielded a value of 5150A for the threshold of the sodium film. These observations indicated that the threshold was the same for all thicknesses of film. Current-voltage curves were obtained for different thicknesses of film, and these were analyzed according to DuBridge's theory. These results also indicate that the work function of the various films is the same. It was concluded from these data that the work function of the freshly distilled aluminum on which the deposits were made was 4.08 volts. This gives a value of 3020A for the threshold of aluminum. It was found that the photoelectric current decreased immediately after forming a film, but that the decrease was much less for films greater than fifty molecular layers. It is suggested that this decrease results from the surface migration of the sodium atoms in forming aggregates.

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