Previous workers have given very little consideration to the thickness of the alkali metal films under investigation. The purpose of the present work was to study the photoelectric properties of the films as a function of their thickness. A molecular beam was used to deposit a computed number of alkali metal atoms on a silver surface cooled with liquid air. In this way, films of various thicknesses were formed and their corresponding spectral response curves taken. The recorded thresholds were arbitrarily chosen as the points, where the currents became definitely measureable. For potassium, the maximum threshold, approximately 5800A, occurred at a film thickness of 3.0 molecular layers while the maximum total photoelectric emission occurred at 12.4 molecular layers, and the photoelectric properties remained constant after 19.0 molecular layers. The atomic spacing in the monomolecular layer is assumed to be the same as that in a solid mass of alkali metal. The threshold for the thick film was approximately 5500A. For rubidium, the maximum threshold, approximately 6200A, occurred at a film thickness of 1.5 molecular layers, while the maximum total photoelectric emission occurred at 5.0 molecular layers, and the photoelectric properties remained constant for film thicknesses beyond 12 molecular layers. The threshold for the thick film was approximately 5900A. For caesium, the maximum threshold, approximately 6600A, occurred at a film thickness of 1.5 molecular layers, while the maximum total photoelectric emission occurred at 5.0 molecular layers, and the photoelectric properties remained constant for film thicknesses beyond 10.0 molecular layers. The threshold for the thick film was approximately 6300A. Some anomalous effects are reported in the case of sodium. The maximum extension of the threshold was found to be much less than that reported by other investigators. A theory is advanced to account for the existence of a maximum in the excursion of the threshold.

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