The photoelectric current from the light emergent side of thin silver films as a function of thickness Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8p58pg97q

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  • This investigation is a study of how the magnitude and energy distribution of the photoelectric current from the light emergent side of thin silver films varies with film thickness. The Fowler-Dubridge graphical analysis of the energy distribution of photoelectrons is used to determine the energy of the most energetic of the emitted photoelectrons, at O°K. The experimental phototube is of the concentric: sphere type. The silver film was deposited on the inside of the sphere to act as a collector for the photoelectrons. The tube was mounted on a vacuum system consisting of a mechanical pump, an oil diffusion pump and an oil vapour trap. During the investigation the tube was left on the pumps at a pressure of approximately 2x10⁻⁷mm. Hg. The photocurrent from the silver film was amplified with a Brown and Dubridge circuit using a FP-54 electrometer tube. The film was irradiated by light from a mercury arc lamp and a Bausch and Lomb monochromator. The intensity of light transmitted through the silver film was monitored by an R.C.A. IP28 electron multiplier phototube. In order to determine the thickness of the silver film a separate experiment was carried out to determine the per cent light transmission through a film of known thickness. The main investigation consisted of depositing a layer of silver on the quartz disc mounted in the center of the experimental tube, and measuring the percentage of transmitted light, of wavelength 2483A, through the film. The emitted photocurrent was then measured as a function of the light intensity at the light emergent side of the silver film, and as a function of the retarding potential across the experimental phototube. The film was increased in nineteen steps to a maximum thickness of 255A, and the above data was recorded for each film thickness. The magnitude of the photocurrent increased with film thickness up to about 40A and was then relatively constant for thicker films. The analysis of the energy distribution of the photocurrent showed an increase of the maximum energy of emission with film thickness up to about 50A. This increase in maximum energy may be due to a change in contact potential between the collector and the silver film or to a decrease in the work function of the silver film as the thickness increases. A decrease in the work function of the silver film, with increasing thickness, would explain the increase in the emitted photocurrent, with increasing film thickness.
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